PW?*^ 


■w^vvuM^iL    .  1 1  u  lu  rd«i^an^f^!«|W(PiM 


QH431 
C377 


^ortI|  Carolina  ^^taie 
College 


This  book  was  presented 

QlH431 


NORTH  CAROLINA  STATE  ;. M  ,'ERSITY  LIBRARIES 


S009607 


3  Q 


J0SEPH.BU2IC|{A 
BOOKBIWOER'S 

BAIT  !  MO^e,  MD. 
GHEEHSBORO.N.C. 
WA3HIN0TeN.0.C. 


This  BOOK  may  be  kept  out  TWO  WEEKS 
ONLY,  and  is  subject  to  a  fine  of  FIVE 
CENTS  a  day  thereafter.  It  is  due  on  the 
day  indicated  below: 


«^rt«l 

22Dec35j 

FE8  -419 

yi 

6Jul42E 

"    FEB25' 

81 

efeb '50^ 

3lJan'562 

^PR  1  -  1! 

ifli 

22May58V 

7i«»'6|P 

jSPR  1  89 

m 

^Ww 

IBPw^  ^  " 

•  *       • 

MflV  ?^) 

2008 

STUDIES  OF  HEREDITY 
m  RABBITS,  RATS,  AND  MICE 


BY 
W.  E/ CASTLE 

RESEARCH   ASSOCIATE   OF   THE   CARNEGIE  INSTITUTION  OF   WASHINGTON 


Published  by  the  Carnegie  Institution  of  Washington 

Washington,  1919 


CARNEGIE  INSTITUTION  OF  WASHINGTON 
Publication  No.  288 


[From  the  Laboratory  of  Genetics  of  the  Bussey  Institution] 


Copied  of  this  Book 
were  first  issued 

JUL  24  \m 


PRESS   OF   J.    B.    LIPPINCOTT   COMPANY 
PHILADELPHIA 


CONTENTS. 


Part  I.    Further    Experiments    upox     the    Modifiability    of     the    Hooded 

Character  in  Rats. 

PAGE 

Selected  races  crossed  with  wild 2 

Part    II.    The    Inheritance    of   White-spotting    in    Rabbits,    with    Special 
Reference  to  Gametic  Contamination. 

Dutch 5 

English 19 

Relation  of  Dutch  to  English 25 

Part  III.    Observations  on  the  Occurrence  of  Linkage  in  Rats  and  Mice. 

Tables 37 

Bibliography 56 


5r?S12 


STUDIES  OF  HEREDITY  IN  RABBITS,  RATS,  AND  MICE. 


By  W.  E.  Castle. 


I.  FURTHER  EXPERIMENTS  UPON  THE  MODIFIABILITY 
OF  THE  HOODED  CHARACTER  OF  RATS. 

In  publications  Nos.  195  and  241  of  the  Carnegie  Institution  of 
Washington  reports  have  been  made  on  the  results  of  a  series  of  experi- 
ments designed  to  show  to  what  extent  a  mendelizing  character,  the 
hooded  pattern  of  piebald  rats,  may  be  altered  by  selection  or  by 
crossing.  At  the  last  report  (Castle  and  Wright,  1916)  the  plus- 
selection  series  had  been  carried  without  out-crossing  through  16 
successive  generations,  in  the  course  of  which  the  mean  grade  of  the 
offspring  had  advanced  from  +2.05  to  +4.13,  in  terms  of  the  arbi- 
trary grading  scale  depicted  in  both  previous  publications.  Since 
then  the  plus-selection  series  has  been  carried  through  four  additional 
generations  of  selection  (17  to  20)  and  the  mean  of  the  selected  race 
has  been  raised  to  4.61.  In  some  respects  this  part  of  the  series  is 
less  satisfactory  than  that  previously  reported  on,  because  smaller 
numbers  of  animals  were  available  from  which  to  select  and  the  selec- 
tion has  therefore  been  less  rigorous.  The  race  has  unmistakably 
fallen  off  in  vigor  and  fecundity  in  later  generations.  It  is  uncertain 
whether  this  should  be  ascribed  to  inbreeding  alone,  uncorrected  by 
selection  for  vigor  (as  in  Miss  King's  experiments),  or  to  increase  in 
the  prevalence  of  disease,  or  to  both  causes.  Certain  it  is,  however, 
that  notwithstanding  increasing  care  in  regard  to  feeding  and  sanita- 
tion a  very  large  proportion  of  our  breeding-pens  in  the  case  of  the 
selected  races  produce  no  young  at  all. 

Tables  1  to  4  show  in  detail  the  grade  distribution  of  the  young 
produced  by  plus-selected  parents  of  generations  17  to  20.  The 
numbers  of  young  produced  in  each  of  these  generations  are  respec- 
tively 351,  420,  280,  and  92.  The  mean  grade  of  the  young  advances 
from  4.13  (in  generation  16)  to  4.48  (in  generation  17),  remains  prac- 
tically stationary  in  generations  18  and  19  at  4.46  and  4.49  respectively, 
and  then  advances  again  (in  generation  20)  to  4.61, 

The  minus-selection  series — which  at  the  last  report  had  been  car- 
ried through  17  generations,  with  an  advance  in  mean  grade  of  the 
young  from  grade  minus  1.00  to  grade  minus  2.70 — has  now  been 

1 

Library 
N.  C,  State  CoUen^e 


2  HEREDITY   IN   RABBITS,    RATS,    AND    MICE. 

carried  through  4  additional  generations  of  selection.  (See  tables 
5  to  8.)  But  this  race  has  shown  even  poorer  vitality  than  the  plus- 
selection  race,  so  that  in  order  to  keep  it  alive  practically  no  young 
could  be  rejected  as  parents  and  consequently  no  further  progress 
has  been  made.  The  numbers  of  young  recorded  for  the  four  addi- 
tional generations  have  been  330,  130,  79,  and  35  respectively,  and 
their  mean  grades  -2.84,  -2.89,  -2.78,  and  -2.74.  The  race  is 
now  practically  stationary  in  grade,  but  seems  likely  soon  to  become 
extinct  despite  our  strongest  efforts  to  keep  it  alive.  Notwithstanding 
the  fact  that  the  race  is  verging  on  extinction  after  long-continued 
close  breeding,  the  variability  of  the  hooded  character  is  still  as  great 
as  ever.  The  standard  deviation  ranges  from  0.25  to  0.45  within 
about  the  same  limits  as  in  the  previous  17  generations  from  minus- 
selected  parents.  In  the  plus-selection  series  the  standard  deviation 
was  also  fully  as  high  in  the  last  generations  as  it  had  been  in  the 
previous  10  generations.  Only  the  initial  7  generations  had  shown 
an  appreciably  higher  variability. 

SELECTED  RACES  CROSSED  WITH  WILD. 

We  may  now  inquire  what  happens  to  the  races  modified  by  selec- 
tion in  opposite  directions,  when  they  are  crossed  with  an  unselected, 
non-hooded  race,  the  wild  race.  This  question  was  considered  in 
some  detail,  so  far  as  the  plus-selected  race  is  concerned,  in  a  previous 
publication  (1916),  where  it  was  shown  that  a  cross  of  the  plus-selected 
race  reduced  the  grade  of  the  hooded  character,  undoing  in  a  measure 
the  work  of  selection.  Selected  animals  which  mated  with  their  like 
should  have  produced  young  of  about  mean  grade  3.75,  actually  pro- 
duced hooded  grandchildren,  extracted  in  F2  from  a  wild  cross,  of 
mean  grade  3.17,  a  falling  off  in  grade  of  over  0.50.  A  second  cross 
with  wild  showed  no  further  falling  off,  but  instead  a  movement  in 
the  reverse  direction  to  3.34  (a  movement  probably  not  significant, 
in  the  light  of  further  experiments).  A  third  cross  with  wild  has 
been  made  on  a  small  scale;  19  hooded  grandchildren  extracted  in  F2 
from  this  third  cross  have  a  mean  grade  of  3.04.  (See  table  9.)  It 
seems  probable,  therefore,  in  the  case  of  the  plus-selected  hooded  char- 
acter, that  the  maximum  efTect  exerted  by  the  residual  heredity  of  the 
wild  race  is  to  reduce  the  hooded  character  in  grade  by  about  three- 
fourths  of  a  grade.  Selection  in  10  previous  generations  had  elevated 
the  grade  of  the  hooded  character  by  about  If  grades.  A  cross  with 
wild  eliminated  less  than  half  of  this  change.  The  remaining  change 
must  be  ascribed  to  changes  effected  in  the  course  of  selection. 

The  minus-selected  hooded  race  has  also  been  crossed  with  this 
same  wild  race.  Originally  of  grade  -1,  it  had  been  altered  by  15 
generations  of  selection  to  the  extent  of  about  1|-  grades,  to  mean 
grade    -2.54.     Females  of  generations  15  to  16  (table  10)  and  of 


MODIFIABILITY    OF   HOODED    CHARACTER  3 

grade   —2.75  were  crossed  with  wild  males  of  the  same  race  used  in 
the  crosses  of  the  plus-selected  race.    The  extracted  hooded  Fo  young 
were  highly  variable,  ranging  in  grade  from   —2.25  to  +3.00,  mean 
—  0.38,  a  remarkable  change  in  the  plus  direction  of  over  2  grades. 

A  second  cross  with  the  wild  race  (table  11)  brought  about  a  further 
movement  of  the  mean  in  a  plus  direction  but  by  a  somewhat  smaller 
amount,  1.39  grades,  the  mean  of  the  twice-extracted  hooded  young 
being  -fl.Ol.     A  third  cross  with  the  wild  race  (table  12)  brought 
still  further  contamination  of  the  hooded  character,  which  now  ceased 
to  vary  below  grade  +1.00,  and  had  a  mean  of  +2.55  in  the  case  of 
over  100  thrice-extracted  hooded  young,  this  being  a  change  in  the  plus 
direction  of  1^  grades.    It  will  be  observed  that  the  hooded  grand- 
children of  0^2068,  table  12,  the  most  plus  in  character  of  the  hooded 
grandparents,  are  very  similar  in  grade  to  the  hooded  young  resulting 
from  three  crosses  of  the  plus-selected  hooded  character  with  the 
same  wild  race  (table  9).    In  other  words,  the  same  wild  race,  when 
its  residual  heredity  is  made  fully  effective  by  repeated  crosses,  brings 
both  the  plus-selected  and  the  minus-selected  hooded  lines  to  a  pheno- 
type  of  common  grade.    This  shows,  contrary  to  my  earlier  opinion, 
that  what  has  really  happened  in  the  case  of  the  selected  races  was 
more  largely  due  to  residual  heredity  than  to  any  change  in  the  gene 
for  the  hooded  character  itself.     ]My  critics  have  been  wrong  when 
they  insisted  that  selection  could  not  change  racial  characters  that 
mendelize  and  change  them  permanently,  and  when  on  this  ground 
they  denied  to  gradual  change  through  selection  an  important  part 
in  the  evolution  of  characters  and  thus  of  races.    But  my  critics  have 
been  right  w^hen  they  insisted  that  evidence  is  wanting  that  change 
in  single  genes  occurs  other  than  spontaneously,  uninfluenced  by  sys- 
tematic selection. 


II   THE  INHERITANCE  OF  WHITE-SPOTTING  IN  RABBITS. 

WITH  SPECIAL  REFERENCE  TO  GAMETIC 

CONTAMINATION.! 

One  of  the  commonest  color  variations  of  mammals  is  white- 
spotting — the  occurrence  of  whollj^  unpigmented  areas  in  the  skin  and 
the  hair  arising  from  it.  Small  unpigmented  areas  are  frequently 
found  in  the  coats  of  wild  mammals,  as,  for  example,  in  the  fur  of  wild 
mice,  rats,  the  common  North  American  rabbit  {Lepus  sylvaticus), 
and  cavies  {e.g.,  Cavia  cutleri).  The  white-spotting  found  in  these 
wild  forms  is  usually  not  extensive.  It  consists  of  a  white  ''star" 
in  the  forehead  or  a  spot  on  the  chest,  or  at  the  end  of  the  tail,  or  on 
a  foot.  Such  locations  of  the  white-spotting  suggest  a  deficiency  of 
pigment  in  the  skin,  either  where  it  closes  together  in  the  median  line 
during  development  of  the  embryo  or  at  the  extreme  limits  of  its 
peripheral  extension  during  development.  At  places  where  the  skin 
regenerates  after  injury,  even  in  self-colored  animals,  a  white  spot 
frequently  develops.  This  is  especially  noticeable  on  the  backs  and 
shoulders  of  horses  where  the  harness  has  ''galled"  them.  That  such 
shght  congenital  deficiencies  of  pigment  as  occur  in  wild  mammals 
are  hereditary  has  been  shown  by  Little  in  the  case  of  the  house- 
mouse,  and  by  Phillips  and  myself  (unpublished  observations)  in  the 
case  of  the  field-mouse,  Peromyscus.  We  observed  in  a  colony  of 
Peromijscus  reared  from  animals  taken  in  Massachusetts  the  occur- 
rence of  individuals  having  tails  partly  or  wholly  white.  This  con- 
dition was  found  to  be  a  Mendelian  recessive  character  in  crosses. 
After  one  or  two  selections  of  white-tailed  individuals,  we  noted  exten- 
sion of  the  white  area  on  to  the  belly. 

In  some  wild  mammals  the  white-spotting  is  more  extensive,  taking 
the  form  of  a  definite  pattern,  as  in  skunks,  the  harp-seal,  and  the 
Malay  tapir.  The  color  pattern  of  skunks,  w^hile  characteristic,  is 
known  to  vary  slightly,  the  value  of  a  pelt  increasing  with  the  amount 
of  black  which  it  contains,  a  fact  which  the  incipient  industry  of 
skunk-farming  in  the  United  States  notes  with  interest.  Selective 
breeding  is  being  directed  toward  the  establishment  of  all-black 
strains  and  no  doubt  it  will  ultimately  be  successful. 

White-spotting  is  so  common  in  the  domestic  animals  as  to  need 
no  comment.  White-spotting  in  more  or  less  definite  patterns  char- 
acterizes the  majority  of  our  breeds  of  cattle,  horses,  dogs,  and  swine. 
Often  the  pattern  is  so  definite  and  so  strongly  inherited  as  to  con- 
stitute a  sort  of  trade-mark  of  breed  purity,  as  in  Hereford  (white- 
faced)  cattle,  Dutch  belted  cattle,  and  Dalmatian  coach-dogs.    Much 

»  Valuable  assistance  in  the  conduct  of  this  investigation  was  given  by  my  former  pupil,  Prof. 
H.  D.  Fish. 


INHERITANCE    OF    WHITE-SPOTTING    IN    RABBITS.  5 

interest  attaches  to  the  inheritance  of  these  patterns  in  crosses,  a 
subject  which  has  been  studied  for  some  years  at  the  Bussey  Institu- 
tion. The  present  paper  will  deal  with  the  subject  of  white-spotting 
in  domestic  rabbits. 

Patterns  of  white-spotting  mendelize  without  known  exception 
but  with  some  irregularity  as  regards  dominance.  In  some  cases 
white-spotting  is  not  expressed  in  the  heterozygote,  but  if  expressed 
at  all  in  the  heterozygote  its  expression  is  always  stronger  yet  in  tlie 
homozygote.  When  the  character  is  nearly  or  quite  suppressed  in 
the  heterozj'gote,  we  may  call  it  recessive;  when  the  character  is 
strongly  expressed  in  the  heterozygote,  we  may  call  it  dominant.  But 
neither  term  is  applicable  without  qualification  in  the  way  that 
recessive  is  applicable  to  complete  albinism  in  rodents. 

With  this  qualification  of  terms,  it  may  be  said  that  there  occur 
among  domestic  rabbits  two  forms  of  white-spotting,  probably  of 
independent  origin  and  certainly  of  quite  different  genetic  behavior, 
since  one  is  recessive  and  the  other  dominant  in  crosses  with  the 
same  race  of  unspotted  rabbits.  The  dominant  form  of  white-spotting 
is  found  in  the  so-called  English  rabbit  and  its  inheritance  has  been 
discussed  by  Castle  and  Hadley  (1915).  The  recessive  form  of  white- 
spotting  is  found  in  Dutch  rabbits,  as  observed  independently  by 
Hurst  and  by  Castle.  Punnett  assents  to  this  conclusion  with  the 
qualification  that  the  inheritance  is  possibly  not  that  of  a  simple 

(one-factor)  sort.  ^,  ,^^, , 

^  '  DUTCH. 

In  September  1910  three  standard-bred  Dutch  rabbits  were  obtained 
from  a  fancier  who  had  bred  and  exhibited  prize-winning  animals 
derived  from  stock  imported  from  England.  They  resembled  grades 
7,  8,  and  9  respectively  (plate  1).  The  female  proved  to  be  sterile 
and  was  ultimately  discarded.  One  of  the  males  (cf3037,  grade  7) 
was  mated  with  two  heterozygous  English  does,  which  produced 
self-colored  blacks  as  recessives  when  mated  with  English  bucks  of 
their  own  race.  For  the  present  we  shall  consider  only  the  non- 
English  young  produced  by  these  matings.  Such  young  would  be  of 
the  same  character  as  those  produced  by  self-colored  ani- 
mals mated  with  Dutch,  since  they  would  arise  from  a  se// 
gamete  furnished  by  the  mother  and  this  would  be  fer- 
tilized by  a  Dutch  gamete  furnished  by  the  father.  Six 
young  of  this  character  were  produced  of  the  accom- 
panying grades. 

This  same  male  was  mated  also  with  3  Himalayan  albino 
does  of  a  race  entirely  free  from  spotting  but  which 
lacked  the  color  factor.  Potentially  these  does  were  self- 
colored.  This  cross  produced  18  young  of  the  accom- 
panying grades.  (See  table  13) 

'  Grade  0  signifies  a  self  animal,  i.e.,  one  without  white  spotting. 


Grade. 

No. 

1 

o 

3 

2 
1 
3 

(See  table  13). 

Grade. 

No. 

•0 
1 
2 

3 

13 

2 

6  HEREDITY   IN   RABBITS,    RATS,    AND   MICE. 

Two  does  derived  from  the  first-mentioned  cross  and  one  derived 
from  the  second  were  employed  in  various  matings  presently  to  be 
described.  The  results  observed  in  the  case  of  all  three  were  so  similar 
that  they  may  conveniently  be  described  together.  It  will  be  borne 
in  mind  that  all  are  Fi  hybrids  between  Dutch  and  self. 

When  crossed  back  with  the  other  original  pure  Dutch  buck  ( cf3036, 
grade  9),  these  three  does  produced  20  young  of  the  grades  shown  in 
table  14.  We  get  here  indications  of  segregation  into  two  groups,  one 
like  the  Fi  mothers  in  grade,  the  other  like  the  Dutch  father,  but  no 
sharp  line  of  division  separates  the  two. 

The  same  three  Fi  females  were  also  mated  successively  with  an 
Fi  male  from  each  of  the  two  crosses  already  described,  with  the 
results  shown  in  table  15.  The  results  are  similar  in  both  cases,  but  it 
will  be  noticed  that  the  lower  grade  Fi  male  (5029,  derived  from  the 
Himalayan  cross)  produced  Fz  young  of  slightly  lower  grade.  The 
F2  range  extends  from  0  to  grade  5  inclusive,  average  1.80.  The  back- 
cross  range  was  from  1  to  7  inclusive,  average  4.60. 

Certain  of  the  F2  young  and  the  back-cross  young  of  grade  4  or 
higher,  which  presumably  would  be  homozygous  for  the  Dutch  char- 
acter, if  it  mendelizes,  were  employed  in  building  up  a  race  of  Dutch 
rabbits  for  further  study.  This  was  done  by  back-crossing  the  selected 
does  a  second  time  with  the  pure  buck,  cf3036,  grade  9,  with  the 
results  shown  in  tables  16  and  17.  Young  were  obtained  which 
ranged  from  grade  1  to  grade  17,  but  which  grouped  themselves  round 
two  modes  situated  at  about  grade  6  and  grade  15  respectively.  We 
shall  presently  consider  the  distribution  further. 

These  same  does  were  also  mated  with  a  male  similar  in  origin  to 
themselves,  \az,  cf5167  (table  18),  a  typical  and  evenly  marked 
Dutch  buck  of  grade  7,  produced  by  the  original  back-cross  (table  14). 
He  bred  in  all  respects  like  his  father  (cf3036,  grade  9)  when  mated 
with  the  same  does,  producing  a  bimodal  group  of  Dutch  young  of 
only  slightly  lower  mean  grade  than  the  young  which  his  father  sired, 
as  might  be  expected  from  the  fact  that  his  grade  was  less  than  his 
father's  grade.     (See  table  18.) 

It  was  now  evident  that  we  had  secured  a  race  of  Dutch  rabbits 
which  produced  only  Dutch  young  and  which  derived  their  Dutch 
character  exclusively  from  the  two  bucks  3036  and  3037,  and  yet 
which  fluctuated  in  grade  around  two  different  modes.  In  fact,  it 
was  soon  discovered  that  the  two  original  Dutch  bucks  were  them- 
selves heterozygotes  of  two  different  types  of  Dutch  pattern  which 
corresponded  with  the  two  modal  conditions  found  among  their 
descendants.  Our  next  task  was  to  isolate  these  two  types  in 
homozygous  form.  This  was  easy  in  the  case  of  the  higher  grade 
(whiter)  type,  which  proved  to  be  recessive.  A  male  of  this  ''white" 
type,  6175,  grade  17,  when  mated  with  females  of  the  same  sort 


INHERITANCE    OF   WHITE-SPOTTING   IN    RABBITS.  7 

produced  only  one  type  of  Dutch  young  varying  around  grade  IG. 
(See  table  19.) 

By  studying  the  results  of  various  matings  of  our  Dutch  does  it 
was  found  possible  to  classify  them  in  three  categories:  (1)  The 
^'white"  type  of  grade  15  to  17,  which  produced  only  the  "white" 
type  when  mated  with  bucks  of  the  same  sort,  as  already  described. 
(See  plate  2,  fig.  19.)  (2)  A  "dark"  type  of  grade  1  to  7,  which  when 
mated  with  bucks  of  the  white  type  produced  no  "white"  offspring, 
but  only  those  of  the  dark  type.  These  mothers  were  evidently  homozy- 
gous "dark,''  the  other  pure  type  (see  plate  2,  fig.  20).  (3)  The  third 
type  of  doe  is  scarcely  distinguishable  from  the  pure  dark  type  except 
by  breeding  test.  It  consists  of  heterozygotes  between  the  two  types, 
a  little  whiter  on  the  average  than  the  dark  type,  but  not  conspicu- 
ously so.  When  they  are  mated  with  "white"  bucks,  young  of  two 
types  are  produced  in  about  equal  numbers,  viz,  heterozygous  darks 
and  pure  whites.  The  original  Dutch  bucks  from  which  the  entire  race 
was  derived  were  both  of  this  heterozygous  dark  type. 

Neither  the  dark  type  nor  the  white  type  isolated  from  this  race  of 
rabbits  conforms  closely  with  the  ideal  Dutch  type  of  the  fancier  (our 
grade  8).  The  one  is  usually  too  dark  and  the  other  too  white.  It 
seems  probable  that  the  fanciers  in  breeding  "prize-winners"  have 
consciously  or  unconsciously  been  producing  heterozygotes,  very  much 
as  in  the  case  of  the  Andalusian  fowl.  Certain  it  is  that  all  the  rabbits 
which  we  have  produced  from  this  stock,  which  would  have  any 
chance  of  winning  a  prize  at  an  exhibition,  have  been  heterozygotes 
between  these  two  types. 

While  the  experiments  with  standard-bred  fanciers'  Dutch  rabbits 
were  in  progress,  and  after  the  white  and  the  dark  types  of  Dutch  had 
been  isolated,  a  third  type  of  Dutch  was  discovered  which  kept  crop- 
ping out  in  a  stock  of  black-and-tan  rabbits  under  observation  for 
another  purpose.  This  stock  was  derived  from  a  single  pure-bred 
black-and-tan  buck  which  had  been  crossed  with  various  other  stocks 
of  rabbits  then  in  the  laboratory.  The  Dutch  pattern  had  been 
introduced  as  a  recessive  character  in  a  certain  j^ellow  rabbit  of 
unknown  pedigree  obtained  by  purchase.  When  the  descendants  of 
this  yellow  rabbit  were  bred  with  each  other,  certain  of  the  young 
produced  were  Dutch  marked.  This  type  of  Dutch  resembled  the 
fanciers'  type  of  Dutch  (grade  8,  plate  1)  so  far  as  the  head  markings 
were  concerned,  but  the  belt  was  very  narrow  and  placed  far  forward 
over  the  shoulders.  (See  plate  2,  fig.  21.)  Because  of  its  origin  within 
the  black-and-tan  stock  we  have  adopted  the  name  "tan"  Dutch  to 
distinguish  it  from  the  other  two  types. 

In  describing  its  variations  we  have  used  the  same  set  of  grades 
(shown  in  plate  1)  which  were  used  in  classifying  the  variations  of 
the  other  two  types,  but  it  must  be  understood  that  rabbits  of  the 


8  HEREDITY   IN   RABBITS,    RATS,    AND   MICE. 

dark  and  of  the  tan  type  which  are  given  the  same  grade  are  not 
exactly  ahke  in  pattern.  The  grade  is  at-^signed  to  express  roughly 
the  total  amount  of  white-spotting  on  the  animal.  A  rabbit  of  grade  3 
in  the  tan  series  will  usually  have  a  whiter  head  with  a  broader  white 
spot  on  the  nose  but  with  a  narrower  collar  than  one  of  the  same 
grade  in  the  dark  series.  With  this  explanation  it  may  be  stated  that 
tan  Dutch  rabbits  bred  with  each  other  have  produced  40  Dutch 
young  with  the  grade  distribution  shown  in  table  21. 

The  variation  is  close  about  grades  3  and  4,  the  race  being  very 
uniform  in  character  for  a  white-spotted  race.  In  origin  it  is  derived 
from  a  single  gamete  which  introduced  the  character  as  a  recessive 
into  the  original  yellow  ancestor. 

Having  now  secured  3  distinct  strains  of  Dutch  rabbits,  it  was  our 
next  task  to  determme  what  were  their  genetic  relationships  to  each 
other,  whether  they  were  allelomorphs  or  due  to  wholly  independent 
factors;  whether  due  to  single  or  to  multiple  genetic  factors,  and 
whether  these  factors  were  constant  or  variable.  Before  these  ques- 
tions can  be  intelligently  discussed  the  variability  of  each  type  by 
itself  must  first  be  known.  That  of  the  tan  race  has  just  been  referred 
to.  It  is  shown  in  table  21.  It  will  be  observed  that  the  mean  grade 
of  the  young  has  a  tendency  to  rise  with  the  grade  of  the  parents. 

The  variation  of  the  uncrossed  ''dark"  race  is  shown  in  table  20. 
The  same  homozygous  buck  (6701,  plate  2,  fig.  20)  was  mated  with  ten 
different  homozygous  dark  does  ranging  in  grade  from  2  to  5.  They 
produced  172  young  ranging  in  grade  from  1  to  7,  mean  3.30.  The 
variation  is  sho\\Ti  graphically  in  text-figure  1,  d.  The  higher-grade 
mothers,  it  will  be  observed,  produce  higher-grade  young,  although 
the  differences  are  not  striking. 

The  variation  of  the  uncrossed  ''white"  race  is  shown  in  table  19 
and  text-figure  1,  w.  Two  bucks  of  grade  17  were  mated  with  8  does 
of  grade  15,  16,  or  17;  they  produced  59  young  with  the  same  range 
of  variation  as  the  mothers  and  of  the  mean  grade  16.25.  Again  we 
observe  a  tendency  for  the  higher-grade  mothers  to  produce  the 
higher-grade  young. 

The  same  homozygous  white  buck  (6175)  which  was  used  in  matings 
recorded  in  table  19  was  mated  also  with  5  homozygous  does  of  the  dark 
race,  with  the  results  shown  in  table  22  and  text-figure  1,  Fi,  D  X  W. 
These  matings  produced  28  young  of  mean  grade  7.28.  All  the  young, 
from  their  parentage,  should  be  heterozygotes  between  white  and  dark 
Dutch,  like  the  original  animals  from  which  these  two  races  were 
isolated.  In  reality  they  agree  closely  with  the  foundation  stock  in 
grade. 

Table  23  shows  the  results  of  matings  of  homozygous  white  bucks 
(one  of  which  was  the  same  individual,  6175,  as  sired  the  young  of 
tables  19  and  22)  with  dark  does  which  were  heterozygous  for  white 


INHERITANCE   OF   WHITE-SPOTTING    IN    RABBITS. 


9 


and  so  were  like  the  foundation  stock.  The  young  tlius  produced 
numbered  130  and  fall  into  two  distinct  groups,  each  varying  about  a 
different  mode.  The  two  groups  apparently  do  not  overlap.  Taking 
the  minimal  class,  grade  12,  as  on  the  line  between  them,  there  are 
65  individuals  below  this  class  and  G4  above  it.    The  former  should  be 


Grade.    0     I 


10     II     M2      13      14     15      16     17 


Text-figure  1. 


heterozygotes  like  the  young  recorded  in  table  22.  Their  mean  grade 
is  very  similar,  7.04  as  against  7.28.  The  latter  are  evidently  homozy- 
gous white  like  the  young  of  table  19.  They  have  a  similar  but  slightly 
lower  mean  grade,  viz,  15.56  as  against  16.25.  This  result  indicates  a 
1  :  1  segregation  of  white  and  dark  Dutch  and  that  accordingly  white 
and  dark  are  allelomorphs.  If  so,  an  ¥■>  generation  should  show 
a  3  :  1   segregation.     To  test  this  point  we  may  summarize   frnm 


10  HEREDITY    IN    RABBITS,    RATS,    AND    MICE. 

tables  16  to  18  all  matings  between  two  heterozygous  parents  (those 
which  produce  both  dark  and  white  young).  In  this  way  we  get  the 
totals  shown  in  table  28,  ''white  X  dark."  Two  groups  of  young  are 
here  shown,  one  dark  (grade  10  or  lower),  the  other  white  (grade  12 
or  higher).  The  numbers  of  individuals  in  these  groups  are  56  and  25 
respectively  (a  rather  poor  3  :  1  ratio);  their  mean  grades  are  5.82 
and  14.40.  (See  text-figure  1,  Fa,  DxW.)  The  dark  group  evidently 
includes  homozygotes  (mode  on  5)  and  heterozygotes  (mode  on  8) 
which  overlap  in  the  intervening  region.  Accordingly  all  facts  thus 
far  noted  indicate  that  white  and  dark  are  allelomorphic  forms  of 
Dutch  marking. 

One  other  test  of  this  hypothesis  is  possible.  Fi  may  be  back- 
crossed  with  the  dark  race.  The  result  of  such  a  test  is  shown  in 
table  29,  "Fi  (white  X  dark)  X  dark,"  and  text-figure  1,  Fi  X  D. 
The  expectation  here  is  the  formation  of  two  groups  of  equal  size, 
homozygous  and  heterozygous  dark,  with  modes  on  3  and  7  respec- 
tively. In  reality  the  intervening  grades  are  the  modal  ones.  This 
does  not  disprove  segregation.  The  flat-topped  variation  curve  ob- 
served is  exactly  what  we  might  expect  from  the  combination  of  two 
simple  variation  curves  which  overlap.  Compare  F2,  Sx  W,  text-figure2. 

The  several  facts  developed  as  regards  the  relation  of  "dark"  to 
"white"  Dutch  pattern  are  presented  graphically  in  text-figure  1. 
The  variability  of  the  uncrossed  races  is  shown  at  the  top;  the  races 
are  distinct,  monomodal,  and  do  not  overlap  in  range.  Immediately 
below  is  shown  the  character  of  Fi.  It  also  is  monomodal  and  it  is 
intermediate.  Next  lower  is  shown  the  variability  of  F2.  The  extracted 
white  race  is  here  seen  to  have  a  lower  mode  than  the  uncrossed  white 
race  and  the  extracted  dark  race  has  a  higher  chief  mode  than  the 
uncrossed  dark  race.  In  other  words,  the  extracted  races  reappear 
in  Fo  in  character  mutually  modified,  although  their  extreme  range  is 
the  same.  The  mode  of  the  heterozygotes  lies  between  the  modes  of 
the  homozygotes,  as  in  Fi  just  above. 

The  results  of  back-crosses  with  each  of  the  parental  races  are 
showTi  in  the  lower  part  of  text-figure  1.  In  the  back-cross  with  the 
white  race  (FiXW),  the  1  :  1  segregation  is  unmistakable;  in  the 
back-cross  with  the  dark  race  (FiXD),  segregation  is  obscured  by  the 
closeness  to  each  other  of  the  modes  for  heterozygous  and  homozygous 
dark  Dutch,  which  results  in  producing  a  composite  flat-topped  curve. 
Everything  indicates  that  dark  and  white  are  simple  allelomorphs,  but 
are  quantitatively  fluctuating  and  mutually  modify  each  other  in 
crosses.  If  they  were  not  allelomorphs,  individuals  which  contained 
neither  form  of  Dutch  should  appear  in  F2.    None  such  is  produced. 

We  may  next  consider  whether  white  Dutch  is  a  simple  allelomorph 
of  self  or  is  composite.  The  pertinent  facts  are  recorded  in  tables  27 
to  29  and  are  shown  graphically  in  text-figure  2.     The  same  white 


INHERITANCE    OF   WHITE-SPOTTING    IN    RABBITS. 


11 


male  (G175)  that  was  crossed  with  dark  females  was  also  crossed  with 
self-colored  (unspotted)  females.  (See  table  27.)  There  wore  pro- 
duced 33  young,  all  showing  a  small  amount  of  white-spotting,  ranging 


Klo. 
,20- 

10- 


^ 
^ 


20- 


10- 


#. 


F,,SxW 


in  grade  from  1  to  3,  mean  1.51  (see  text  figure  2,  Fi,  SxW).  F2  is 
somewhat  puzzling  in  character.  (See  table  28  and  text-figure  2.) 
Nearly  25  per  cent  of  the  191  young  recorded  are  extracted  selfs,  but 


12 


HEREDITY   IN   RABBITS,    RATS,   AND   MICE. 


if  we  take  the  whitest  48  individuals  to  be  extracted  "whites"  it  is 
evident  that  they  are  considerably  modified  from  the  condition  of  the 
uncrossed  race,  since  their  mode  Ues  at  12  to  14,  not  at  16  to  17,  and 
the  highest  grade  (17)  of  the  uncrossed  white  race  does  not  reappear 
at  all.  The  back-cross  of  Fi  with  white  must  be  investigated  before 
one  can  interpret  the  F2  result  with  confidence.  But  the  back-cross 
(table  29  and  text-figure  2,  Fi  X  W)  makes  it  very  clear  that  segrega- 
tion occurs  on  a  1  :  1  basis.  The  116  young  thus  produced  fall  into 
two  groups  which  do  not  overlap  and  each  of  which  is  monomodal. 
Each  contains  58  individuals.  But  the  ex- 
tracted groups  show  mutual  modification.  The 
mode  of  the  lower  group  is  not  at  grade  1,  as 
in  Fi,  but  at  grades  4  to  6,  while  the  mode  of 
the  upper  group  is  not  at  16  to  17,  as  in  the 
uncrossed  white  race,  but  at  15. 

The  back-cross  of  Fi  with  self  (table  29  and 
text-figure  2,  Fi  X  S)  gives  a  variation  nearly 
covering  the  combined  range  of  self  and  Fi,  as 
expected,  but  the  two  expected  groups  (if  they 
are  distinct)  lie  so  close  together  that  it  is 
impossible  to  separate  them. 

The  several  facts  developed  in  crosses  of 
white  with  self  indicate  that  white  is  a  simple 
allelomorph  of  self.  If  so,  and  if  dark  is  also 
an  allelomorph  of  white,  as  indicated  by  text- 
figure  1,  then  self  and  dark  should  be  allelo- 
morphs of  each  other.  Such  is  probably  the 
case,  but  crosses  of  self  with  dark  are  incon- 
clusive because  the  two  conditions  are  so 
close  to  each  other  on  the  grading  scale  that 
it  is  difficult  to  demonstrate  segregation.  See 
tables  27  and  29  and  text-figure  3.  This  figure 
shows  (at  the  top)  the  variation  of  uncrossed  dark  in  relation  to  self. 
Below  is  shown  the  variation  of  Fi,  self  being  usually  dominant.  A 
back-cross  with  dark  (Fi  X  D)  produces  less  than  the  expected  propor- 
tion of  self  (nearly  50  per  cent  as  indicated  by  Fi)  and  produces  extracted 
darks  of  lower  mean  grade  than  the  uncrossed  darks.  This  is  evidence 
of  mutual  modification  of  self  and  dark  in  the  heterozygote,  so  that 
they  emerge  in  the  gametes  modified.  So  far,  then,  the  evidence  indi- 
cates that  self,  dark,  and  white  are  all  allelomorphs,  but  that  they 
fluctuate  quantitatively  and  mutually  modify  each  other  when  asso- 
ciated in  heteroz3'gotes. 

We  may  now  consider  the  relation  of  these  three  to  tan  Dutch.  If 
tan  is  an  allelomorph  of  either  of  the  other  forms  of  Dutch,  it  should 
be  an  allelomorph  of  all  three  conditions,  in  fact  sl  fourth  allelomorph. 


Grade    0       1       2      3      4     5      6     7 

Text-figure  3. 


INHERITANCE    OF    WHITE-SPOTTING    IN    RABBITS. 


13 


No. 
20- 


10  ■ 


10- 


0^ 


[^^^ 


F,,TxW 


20 


10 


<w 


To  test  this  matter,  crosses  of  tan  have  been  made  with  each  of  the 
other  three,  white,  dark,  and  self.  Tan  was  found  to  be  allohjmorphic 
with  self  in  the  black-and-tan  race  where  it  appeared  (table  12).  but 
self  being  a  purely  negative  term  (meaning  as  here  used  unspotted) 
one  could  not  be  sure  in  advance  that  self,  as  the  allelomorph  of 
different  kinds  of  Dutch,  would  be  one  and  the  same  thing  in  all 
cases.  This  would  require  demonstration.  It  is  necessary,  then,  to 
ascertain  first  whether  tan  Dutch  is  an  allelomorph  of  white  and  of 
dark. 

Crosses  of  tan  v>ith  white  have  given  the  results  shown  in  tables  26, 
28,  and  29,  and  also  graphically  in  text-figure  4.  Fi  is  intermediate. 
F2  is  likewise  interme- 
diate, but  varies  to  or 
into  the  range  of  the 
uncrossed  races  with 
indications  of  segre- 
gation of  modified  tan 
and  modified  white 
individuals.  The  ab- 
sence of  selfs  in  Fo 
shows  white  Dutch 
and  tan  Dutch  to  be 
allelomorphs.  The 
back-cross  with  white 
(FiXW)  produces  in- 
dividuals varying  (as 
expected)  all  the  way 
from  the  Fi  to  the  un- 
crossed  white  race, 
but  with  an  apparent  q. 
tendency  to  form  ^^^^« 
modes  on  10  and  15. 
These,  it  seems,  represent  the  mutually  modified  modal  conditions  of 
the  Fi  and  the  white  race.  The  extracted  whites  have  tlieir  mode  low- 
ered to  15,  instead  of  on  16  or  17,  as  in  the  uncrossed  white  race;  and 
the  extracted  Fi's  have  their  mode  advanced  from  grade  7  (in  tlie  orig- 
inal Fi's)  to  grade  10. 

The  races  dark  and  tan  are  similar  to  each  other  in  grade  (see  text- 
figure  5),  but  differ  in  the  location  of  their  unpigmented  areas,  as 
already  explained.  Dark  has  a  wider  collar  and  a  darker  head;  tan 
has  a  narrower  collar  and  a  whiter  head.  Fi  is  nearly  or  completely 
self-colored,  since  dark  tends  to  make  the  head  pigmented  and  /<in  to 
make  the  collar  pigmented.  Consequently  there  is  little  sjxice  left 
unpigmented.  Fo  is  quite  variable,  some  individuals  being  darker 
than  either  uncrossed  race,  while  others  are  whiter.    The  range  is  from 


20 


10 


^^:;^^^^^?Er^ 


F,   xW 


.Ji?M$tet 


5     6     7      8     9     10 
Text-figure  4. 


11      12 


14  HEREDITY   IN   RABBITS,    RATS,    AND   MICE. 

grade  0  to  grade  11.  (Text-fig.  5,  F2,  DxT.)  Indications  of  segregation 
are  plainly  seen  in  F2,  but  it  is  impossible  to  be  sure  of  the  number  of 
factors  involved  because  dark  and  tan  are  so  similar  in  grade.  But  we 
know  that  dark  is  an  allelomorph  of  white.  If  it  is  also  an  allelomorph 
of  tan,  then  Fi  individuals  should  produce  dark  gametes  and  tan  game- 
tes in  equal  numbers.  These,  in  a  cross  of  Fi  with  white,  uniting  with 
white  should  produce  two  kinds  of  heterozygotes :  (a)  white-dark  hete- 
rozygotes  like  those  of  text-figure  1,  Fi,  and  (6)  white-tan  heterozygotes 
Hke  those  of  text-figure  4.  The  former  range  from  grade  5  to  grade  11, 
the  latter  from  grade  6  to  grade  9.  The  ranges  are  similar  and  the 
mode  of  each  group  is  on  grade  7.  Mutual  modification  of  dark  and 
tan  would  tend  to  extend  the  range  of  segregates  in  both  directions, 
as  observed  in  Fo  (text-figure  5). 

The  observed  back-cross  generation  (table  29  and  text-figure  5, 
Fi  X  W)  is  distinctly  bimodal,  as  the  hypothesis  just  formulated 
(that  dark  and  tan  are  allelomorphs)  would  demand.  Indeed,  the 
evidence  of  1  :  1  segregation  are  clearer  than  we  should  expect,  but 
the  modes  of  the  expected  groups  are  farther  apart  than  we  should  have 
expected.  (See  text-figure  5.)  The  lower  mode  is  at  4,  not  at  7  as  in 
the  Fi  produced  by  dark  crossed  with  white;  and  the  upper  mode  is 
at  12  to  14,  not  at  7,  as  in  the  Fi  of  tan  crossed  with  white.  It  appears, 
therefore,  that  while  dark  segregates  from  tan  in  the  gametes  formed 
by  Fi  individuals,  each  segregates  in  an  altered  form,  the  dark  having 
become  darker  and  the  tan  lighter  as  a  result  of  their  association  in 
the  heterozygous  Fi.  This  is  indicated  both  by  the  bimodal  condition 
and  increased  range  shown  by  the  cross  of  Fi  with  white,  as  just  described, 
and  also  by  the  wide  range  of  the  F2  from  the  cross  of  dark  with  tan. 

An  alternative  hypothesis,  which  has  been  given  careful  considera- 
tion and  which  in  fact  the  cross  of  Fi  with  white  was  especially  designed 
to  test,  is  this:  that  dark  and  tan  are  due  to  independent  factors  and 
that  the  two  together  constitute  white.  With  this  hypothesis  the 
following  facts  harmonize :  Dark  crossed  with  tan  produces  in  Fi  and 
F2  individuals  which  are  self-colored  {i.e.,  which  have  no  white-spotting) 
as  well  as  others  which  are  whiter  than  either  the  dark  or  the  tan 
race.  The  former  may  be  interpreted  as  those  which  lack  (or  are 
heterozygous  for)  both  dark  and  tan;  the  latter  as  those  which  have 
both  dark  and  tan.  But  it  should  be  observed  that  none  of  the  275  Fo 
young  which  have  been  produced  extend  into  the  range  of  the  uncrossed 
white  race,  where  about  25  per  cent  of  them  should  lie  if  the  hypothesis 
is  correct.     Compare  text-figures  1  and  5. 

Another  way  of  testing  the  hypothesis  that  dark  and  tan  are  due 
to  independent  Mendelian  factors  is  the  cross  of  Fi  with  white.  If  the 
hypothesis  is  correct,  Fi  individuals  should  produce  four  kinds  of 
gametes,  viz,  those  which  will  transmit  (a)  both  dark  and  tan,  (6) 
dark  alone,  (c)  tan  alone,  and  (d)  neither  dark  nor  tan.    By  the  hypoth- 


INHERITANCE    OF   WHITE-SPOTTING    IX    RABBITS. 


15 


esis  white  is  dark  plus  tan.  Therefore,  white  crossed  with  Fi  should 
produce  four  kinds  of  zygotes:  (a)  dark  and  tan  united  witli  white 
(dark  and  tan),  equivalent  to  homozygous  white;  (6)  dark  united 


No 
50- 

40 

30- 

20- 

10- 


0 

20- 

10- 


10- 


3T 


:^>rv^ 


0- 
90- 

SO- 
TO' 
60- 


50- 


40- 


30- 


20- 


10- 


?^^^<Kq 


F,,DxT 


Fz.  DxT 


50i 


4.0- 


30' 


20- 


10- 


20' 
10- 


10- 


\^ 


^ 


20- 


10- 


10- 


20' 


10- 


\1'^ 


F,,DxT 


7^ 


31 


F,  xT 


U^l^^^fe^ 


N 


"^r^ 


F,  xO 


^^:i 


F,  xW 


^ 


F^    F>S\ 


-f^ 


\' 


iL. 


Grade    0      I      2     3      4     5      6      7      8     9     10    II  0      I      2     3      4     5      6      7     6      9     10     11     12    13     14     15    16    17 

Text-figure  5. 

with  white;  (c)  tan  united  with  white;  and  (d)  self  (neither  dark  nor 
tan)  united  with  white.  The  observed  modal  values  of  these  four 
kinds  of  combinations  are  respectively,  in  terms  of  grades:  (a)  10 


16  HEREDITY   IN   RABBITS,    RATS,   AND   MICE. 

(text-figure  1,  W);  (6)  7  (text-figure  1,  Fi);  (c)  7  (text-figure  4,  Fi); 
and  (d)  1  or  2  (text-figure  2,  Fi).  That  is,  this  cross  should,  under  the 
hypothesis  considered,  produce  a  trimodal  figure,  with  a  mode  for 
25  per  cent  of  the  individuals  at  either  end  of  the  grading  scale  and 
with  a  still  larger  mode  (for  50  per  cent  of  all  individuals)  at  an  inter- 
mediate point.  But  what  is  actually  observed  is  very  different.  The 
figure  (text-figure  5,  Fi  X  W)  is  not  trimodal  but  bimodal,  and  neither 
of  the  modes  is  where  the  hypothesis  demands  that  modes  should  be. 
This  is  conclusive  evidence  against  the  correctness  of  the  hypothesis 
in  question,  but  is  entirely  in  harmony  with  the  alternative  one,  that 
dark  and  tan  are  allelomorphs  but  segregate  in  modified  form,  one 
on  the  whole  darker,  the  other  on  the  whole  lighter  than  before  they 
were  crossed  with  each  other. 

This  matter  of  modification  on  crossing  is  one  deserving  further 
consideration.  It  is  in  evidence  in  all  the  crosses  made.  It  is  clearest 
where  the  races  crossed  differ  most  in  grade,  and  it  seems  to  consist 
in  a  partial  obliteration  of  those  differences.  Thus  the  uncrossed 
white  race  has  its  mode  at  17,  the  self  race  at  0.  (See  text-figure  2.) 
In  F2  the  extracted  whites  contain  no  individual  as  high  in  grade  as 
17,  and  the  highest  mode  lies  at  12  to  14,  facts  which  indicate  that 
white  has  been  lowered  in  grade  by  its  association  with  self  in  the  Fi 
zygotes.  The  back-cross  of  Fi  with  white  also  shows  modification  but 
intermediate  in  amount,  as  might  be  expected  from  the  fact  that,  in 
the  case  of  each  zygote  formed,  only  one  of  the  two  conjugating 
gametes  had  been  subjected  previously  to  modifying  influences,  viz, 
that  one  which  was  furnished  by  the  Fi  parent.  The  mode  in  this 
case  lies  at  15,  instead  of  at  17,  as  in  the  uncrossed  race,  or  at  12  to  14, 
as  in  the  F2  extracted  whites. 

That  the  extracted  white  has  less  influence  in  whitening  an  Fi  zygote 
than  the  uncrossed  white  is  shown  further  by  a  comparison  of  Fi  self  X 
white  (text-figure  2)  with  the  back-cross  of  Fi  with  self.  Fi  had  its 
mode  at  grade  1  and  ranged  upward  to  grade  3,  but  contained  no  self 
individuals.  If  extracted  white  were  identical  with  uncrossed  white  in 
its  whitening  influence,  then  in  the  back-cross  with  self  half  the  zygotes 
should  be  of  grade  1  or  higher.  But  in  the  observed  back-cross  less 
than  one-third  of  the  zygotes  show  any  white,  viz,  29  out  of  94,  and  these 
are  lower  in  grade  than  the  original Fi's,  viz,  1.10  as  compared  with  1.51. 

The  self  character  also  emerges  modified  after  the  cross  with  white. 
For  the  Fi  zygotes,  consisting  of  pure  self  united  with  pure  white 
(text-figure  2,  Fi,  S  X  W)  were  all  close  to  self  in  grade,  with  a  mode 
on  grade  1  and  ranging  upward  only  to  grade  3.  But  the  zygotes 
formed  by  extracted  self  united  with  pure  white  produced  in  the  back- 
cross  of  Fi  with  white  (text-figure  2,  Fi  X  W)  range  in  grade  from  1  to 
9  -with  a  broad  low  mode  at  4  to  7.  Evidently  they  have  been  much 
increased  in  grade  in  the  direction  of  white. 


INHERITANCE    OF   WTIITE-SPOTTING    IN    RABBITS.  17 

Again,  in  the  crosses  of  dark  witli  wliite  (text-figure  1),  we  sec  tlie 
mutual  modification  of  the  contrasted  conditions  taking  place.  Un- 
crossed dark  has  its  mode  on  grade  3,  and  uncrossed  white  on  grade  17. 
Extracted  white  as  seen  in  F2  has  its  mode  on  grade  15,  but  in  the  back- 
cross  with  pure  w^hite  it  has  its  mode  on  the  intermediate  grade,  16. 

As  regards  the  modification  of  dark,  it  will  be  observed  that  the 
original  Fi  individuals  are  of  lower  a\erage  grade  than  tlie  lower  group 
of  individuals  produced  by  the  back-cross  with  white.  The  mode  of 
the  former  also  is  on  grade  7,  that  of  the  latter  is  on  grade  8.  The  one 
consists  of  pure  dark  united  with  pure  white,  the  other  of  extracted 
dark  united  with  pure  white.  The  extracted  dark  has  evidently  been 
whitened,  exactly  as  extracted  white  has  been  darkened. 

Uncrossed  tan  and  uncrossed  white  have  their  modes  on  3  and  17, 
respectively  (text-figure  4).    The  mode  of  Fi  is  on  grade  7,  but  the 
mode  of  extracted  tan  united  with  white  is  on  grade  10,  as  seen  in  the 
back-cross  of  F:  with  white.    This  shows  that  extracted  tan  has  been 
whitened  as  compared  with  uncrossed  tan.     That  white  has  had  its 
grade  lowered  by  the  cross  with  tan  is  also  shown  in  the  back-cross 
of  Fi  with  white.    Its  mode  lies  at  15,  not  at  17  as  in  uncrossed  white. 
If,  in  the  several  cases  considered,  crossing  tends  to  mutual  modi- 
fication and  assimilation  to  each  other  of  the  contrasted  conditions 
brought  together  in  the  cross,  why  does  crossing  of  dark  with  tan 
extend  rather  than  shorten  the  range  of  variation,  producing  in  F2 
individuals  darker  than  either  uncrossed  race  and  others  lighter  than 
either  uncrossed  race?    The  answer  to  this  question  is  perhaps  to  be 
found  in  the  imperfection  of  our  scale  of  grades.    The  scale  is  a  linear 
one,  whereas  the  variation  is  not  entirely  linear;  for  dark  Dutch  and 
tan  Dutch  have  the  same  modal  grade,  3,  yet  are  different  in  somatic 
character,  as  has  already  been  stated.   Tan  Dutch  has  a  white  head  and 
narrow  collar,  dark  Dutch  has  a  dark  head  and  wider  collar.    In  Fi,  if 
pigment  simply  dominates  whiteness,  we  may  expect  to  get  a  dark  head 
and  a  narrow  collar  simultaneously,  i.  e.,  a  condition  with  less  white 
than  either  parent  possessed,  which  in  general  is  the  result  observed. 
Since  uncrossed  dark  Dutch  varies  down  to  grade  1  and  uncrossed 
tan  down  to  grade  2,  an  extension  of  the  dark  areas  due  to  crossing 
naturally  carries  the  pigmentation  in  Fi  down  to  grade  0  (self)  in  a 
certain  percentage  of  cases   (1  in  6  observed).     See  text-figure  5. 
In  F2  the  percentage  of  selfs  is  still  larger,  being  about  1  in  3.    There 
are  also  found  in  F2  whiter  individuals  than  either  uncrossed  race 
contained.    How  these  have  arisen  is  indicated  by  the  back-crosses  of 
Fi  with  dark  and  with  tan  respectively.     The  back-cross  with  dark 
(text -figure  5,  Fi  X  D)  produces  a  monomodal  group  closely  resembling 
the  Fi  group,  but  with  slightly  higher  range.     This  shows  that  the 
potent  factor  in  lowering  the  Fi  range  was  the  dark  gamete,  since  it 
is  the  only  common  factor  entering  into  both  crosses.    On  the  other 


18  HEREDITY   IN    RABBITS,    RATS,    AND    MICE. 

hand,  the  back-cross  with  tan  (Fi  X  T)  shows  bimodal  variation  with 
the  modes  at  0  and  5  respectivelj^  This  indicates  that  the  gametes 
formed  by  Fi  are  really  of  two  types,  extracted  dark  and  extracted 
tan.  The  former  uniting  with  pure  tan  produces  a  group  like  Fi  but 
apparently  of  even  slightly  lower  grade,  since  the  modal  condition  is 
now  0,  not  1.  The  extracted  tan  gametes  uniting  with  pure  tan  pro- 
duce a  group  like  pure  tan,  but  of  apparently  higher  grade,  since  the 
mode  is  now  on  5,  not  3  as  in  pure  tan.  Hence  extracted  tan  is  poten- 
tially of  higher  grade  than  pure  tan,  a  conclusion  supported  by  F2 
from  dark  crossed  with  tan,  for  here  we  observe  that  extracted  tan 
meeting  extracted  tan  produces  zygotes  of  grade  5  to  8  or  even  higher, 
whereas  pure  tan  does  not  exceed  grade  5.  Yet,  to  return  to  the  imper- 
fection of  our  grading  scale,  these  higher  grades  consist  merely  in 
combining  a  wider  collar  with  the  same  form  of  head  markings  as 
are  found  in  the  tan  series  of  grade  4  or  5.  Hence  it  appears  that  tan 
is  regularly  modified  through  its  contact  with  dark  in  an  Fi  zygote  in 
the  way  of  acquiring  a  wider  collar,  whereas  dark  is  modified  by  the 
same  agency  in  the  waj^  of  acquiring  a  narrower  collar.  Yet  there  is 
no  indication  that  head  marking  and  collar  marking  are  due  to  dis- 
tinct single  genetic  factors,  but  merely  that  they  are  qualitatively 
different.  This  difference  tends  to  disappear  through  mutual  influ- 
ence in  the  heterozygous  condition,  but  the  difference  disappears  more 
rapidly  in  collar  markings  than  in  head  markings;  hence  the  extended 
range  of  grades  in  F2.  Because  their  collars  become  more  alike  the 
extracted  darks  rank  lower  in  grade  and  the  extracted  tans  rank 
higher;  for  it  will  be  recalled  that  the  uncrossed  darks,  though  having 
wider  collars  than  the  tans,  were  graded  lower  on  account  of  their 
dark  heads.  The  collar  changes,  then,  are  actually  blending  in  this 
cross,  as  in  all  the  others  studied,  but  give  the  appearance  of  segrega- 
tion with  differences  emphasized,  merely  because  of  the  inadequacy 
of  our  linear  grading  scale  to  record  simultaneously  changes  in  head 
and  in  collar  markings  when  these  occur  with  unequal  rapidity. 

It  has  already  been  shown  that  we  have  conclusive  evidence  that 
Fi,  from  the  cross  of  tan  with  dark  (text-figure  5),  produces  two  types  of 
gametes,  not  four  types,  this  evidence  being  (a)  the  bimodal  variation 
seen  in  the  back-cross  of  Fi  with  tan  and  (6)  the  bimodal  variation 
seen  in  the  cross  of  Fi  with  white.  These  results  indicate  that  tan 
and  dark  are  to  be  regarded  as  allelomorphic  but  mutually  modifying 
conditions,  as  had  already  been  found  to  be  true  for  tan  and  white, 
for  dark  and  w^hite,  and  for  self  and  w^hite.  We  have,  then,  a  condition 
of  multiple  allelomorphs  in  white-spotting  patterns  of  Dutch-marked 
rabbits,  which  includes  the  forms  self,  dark  Dutch,  tan  Dutch,  white 
Dutch,  and  possibly  many  other  types  or  conditions  of  white-spotting 
which  with  suflSciently  accurate  observation  might  be  distinguished 
from  each  other. 


INHERITANCE   OF   WHITE-SPOTTING    IN   RABBITS.  19 

The  foregoing  observations  show  unniistakal)ly  that  tlie  several 
members  of  this  allelomorphic  series  tend,  as  a  result  of  crosses,  to 
become  more  Hke  each  other.  This  has  been  described  as  mutual 
modification,  but  it  should  be  expressly  stated  that  in  the  light  of 
our  experiments  with  rats  ''modification"  need  not  be  regarded  a.s 
change  in  the  nature  of  a  single  gene,  but  merely  as  equalization  of  the 
residual  heredity  additional  to  the  single  genes  which  produce  mono- 
hybrid  ratios. 

ENGLISH. 

In  November  1909  there  were  received  at  the  Bussey  Institution 
four  "English"  rabbits,  1  male  and  3  females,  bred  by  R.  W.  Wills, 
of  Hornerstown,  New  Jersey.  In  terms  of  the  grading  scale  shown  in 
plate  3,  the  male  was  of  grade  2^;  the  females  were  of  grades  2,  2f , 
and  3,  respectively. 

In  matings  of  the  male  with  each  of  the  3  females,  there  were  pro- 
duced both  English  and  self-colored  young,  as  shown  in  table  30; 
of  the  former,  21 ;  of  the  latter,  8.  The  self  young  were  later  found  to 
produce  no  Enghsh  young  when  bred  inter  se.  Hence  it  seems  clear 
that  English  is  a  dominant  jNIendelian  character,  that  self  is  recessive 
in  relation  to  it,  and  that  the  4  English  parents  were  all  heterozygous 
dominants. 

The  question  now  arose  whether  homozygous  English  rabbits  could 
be  produced  and  why  English  rabbits  were  not  regularly  bred  in 
homozygous  form.  We  did  not  have  long  to  wait  for  an  answer  to 
these  questions.  Table  30  shows  that  the  English  young  of  our  4 
original  English  rabbits  fall  into  two  groups  quite  different  in  appear- 
ance. Of  the  15  3"oung  which  were  graded,  5  were  of  grade  1  or  1^, 
while  10  were  similar  to  the  parents  in  grade,  varying  from  grade  2 
to  3.  The  group  of  low-grade  English  was  found  to  consist  of  homo- 
zygous individuals  which  produced  only  English  young  in  crosses 
with  each  other  or  with  selfs.  The  higher-grade  group,  twice  as  num- 
erous in  individuals,  was  found  to  consist  of  heterozygotes.  These 
are  preferred  by  the  fancier  because  of  their  much  more  striking  color- 
pattern.  The  homozygote  is  in  appearance  only  an  impure  white 
animal,  but  the  heterozygote  is  beautifully  mottled.  It  is  therefore 
clear  why  the  fancier  breeds  heterozygotes.     (See  plate  3.) 

Our  original  English  buck,  2545,  was  also  mated  with  self-colored 
does  of  several  different  sorts,  \az,  gray,  cream,  yellow,  sooty  yellow 
(tortoise),  black,  and  black-and-tan.  In  regard  to  color  inheritance, 
these  matings  gave  us  such  results  as  are  already  familiar  through 
earlier  publications  by  Punnett,  Hurst,  and  ourselves.  We  may 
therefore  confine  our  attention  to  the  behavior  of  the  English  pattern 
in  crosses.  Table  30,  b,  shows  the  results  obtained;  20  English  and 
IS  self  young  were  recorded  from  these  matings.  No  grade  was 
recorded  for  17  of  the  English  young.     The  others  varied  in  grade 


20 


HEREDITY   IN   RABBITS,    RATS,    AND    MICE. 


from  U  to  4,  the  mean  being  2.80.  Evidently  the  heterozygous 
English" produced  by  these  matings  with  unrelated  does  were  much 
more  variable  than  those  produced  by  matings  of  the  original  English 
individuals  with  each  other. 

It  now  occurred  to  us  to  see  to  what  extent  this  variabiUty  could 
be  carried  farther  in  a  plus  direction  by  selection.  Accordingly  we 
chose  the  grade  4  individual  produced  by  9  1492  as  the  starting-point 
of  the  experiment.  This  individual  had  been  recorded  as  d'2711. 
He  constitutes  generation  1  of  the  selection  experiment  now  to  be 
described,  all  animals  produced  in  those  experiments  deriving  their 
English  from  him.  He  was  mated  with  a  black-and-tan  doe,  with 
three  black  does,  and  six  Himalayan  (albino)  does,  all  free  from 
racial  white  spotting  (EngHsh  or  Dutch).  These  matings  produced 
56  young  (table  31,  b)  equally  divided  between  heterozygous  English 
and  self.  The  English  young  were  of  higher  grade  than  the  English 
young  produced  by  0^2545.  (Compare  tables  30  and  31.)  They 
ranged  from  grade  3  to  grade  5,  mean  3.89,  as  compared  with  a  mean 
of  2.80  for  the  young  of  cf  2545,  produced  in  similar  matings. 

Male  2711  was  later  mated  to  5  of  his  heterozygous  English  daugh- 
ters produced  in  the  matings  already  described,  and  also  to  one  of  the 
resulting  grand-daughters.  The  character  of  their  young  is  shown 
in  table  31,  a.  The  mothers  form  generations  1^  and  2^  of  the  selected 
English  race.  They  vary  in  grade  from  3|-  to  5.  They  produced  three 
classes  of  young:  low-grade  English  (homozygotes),  high-grade 
English  (heterozygotes),  and  selfs.  Their  respective  numbers  were 
12,  11,  and  8.  The  mean  of  the  low-grade  English  group  was  1.52, 
that  of  the  high-grade  group  was  3.93,  which  agrees  closely  with  the 
grade  of  the  same  group  produced  by  matings  with  non-English  (self) 
does  (table  31,  b). 

Grouping  the  mothers  by  grade,  the  relation  shown 
herewith  is  observed  between  grade  of  mother  and 
grade  of  young.  This  indicates  that  selection  of 
higher-grade  parents  would  probably  result  in  produc- 
ing higher  grade  young. 

A  son  of  0^2711,  viz,  cr5086,  generation  1^,  grade  4|-,  was  chosen 
to  succeed  him  in  the  selection  experiment.  This  buck  was  mated 
with  10  different  heterozygous  English  does,  5  of  which  had  also  been 
mated  with  his  father.  (See  table  32.)  There  resulted  20  homozygous 
English  young,  44  heterozygotes,  and  35  selfs.  The  mean  grade  of  the 
homozygotes  was  1.38,  that  of  the  heterozygotes  was  3.96,  averages 
not  very  different  from  those  which  characterized  the  young  of  d^2711 
(table  31).  Consequently  no  advance  can  be  claimed  as  a  result  of 
the  selection  of  cf5086.  He  was  mated  also  with  3  black-and-tan 
does  (table  32,  b),  producing  thus  10  English  j^oung  of  mean  grade 
3.15,  a  lower  average  than  that  given  by  cf  2711  in  matings  with  self 


Mother. 

Mean  of 
young. 

3.50 
4.75 
5.00 

3. S3 
3.80 
4.25 

INHERITANCE    OF   WHITE-SPOTTING    IN    RABHITS.  21 

does,  but  it  must  be  borne  in  niind  that  the  mothers  were  nf)t  all 
identical  in  the  two  cases.  Black-and-tan  does  seemed  in  general  to 
give  lower-grade  oftsjiring  than  does  of  the  self  black  and  Himalayan 
races,  with  which  cf  2711  had  been  mated. 

We  next  used  as  sire  in  the  selection  experiment  cfoSTo,  generation 
2^,  grade  4|,  a  son  of  cf  5086  by  his  half  sister.  (8ee  table  33.)  He 
was  mated  with  9  different  heterozygous  English  does,  all  but  one  of 
which  had  also  been  mated  with  his  father.  They  produced  English 
young  of  somewhat  lower  mean  grade  than  they  had  borne  bv  the 
father,  6^5086.  (See  tables  32  and  33.)  17  homozygous  English 
young  were  of  mean  grade  1 .20;  29  heterozygous  English  young  were  of 
mean  grade  3.79;  there  were  also  25  self  young.  Again  the  higher- 
grade  mothers  produced  the  higher-grade  young.  Hence  there  wa.s 
evidently  material  favorable  for  selection  among  the  mothers,  if  not 
among  the  fathers.  This  male  was  now  discarded  and  replaced  by 
an  own  brother  of  slightly  higher  grade,  viz,  cf  5555,  generation  2^, 
grade  4f .     (See  table  34.) 

This  male  (cf5555)  was  bred  more  extensively  than  any  of  his 
predecessors  and  produced  higher-grade  offspring.  He  was  mated  to 
the  same  does  as  his  father  and  grandfather  and  also  to  a  number  of 
new  ones  which  now^  became  available.  By  all  classes  of  does  he  pro- 
duced higher-grade  3'oung  than  had  an}-  of  his  predecessors.  He  also 
produced  higher-grade  young  by  his  mates  of  higher  grade  than  by 
his  lower-grade  mates.  (Table  34.)  In  his  case,  then,  a  second 
advance  had  been  made  by  selection  in  the  male  line  and  the  necessary 
variation  was  evidently'  present  to  make  possible  similar  advances  by 
selection  in  the  female  line.  JNIale  5555  produced  41  homozygous 
English  young,  125  heterozj-gous  English,  2  ungraded  English,  and 
65  selfs,  or  all  together  168  English  and  65  selfs. 

The  next  sire  tested  was  a  son  of  cf5555;  viz,  6^6370,  grade  5, 
generation  3,  as  regards  selected  ancestry.  (See  table  35.)  He  was 
more  advanced  in  grade  and  generations  than  any  male  thus  far 
tested  and  produced  higher-grade  young  by  females  of  the  same 
grade.  Many  of  the  older  females  had  now  been  discarded,  but 
enough  remained  to  form  a  standard  of  comi:)arison  between  the 
genetic  properties  of  this  male  and  those  of  his  predecessors.  The 
mean  grade  of  the  heterozygous  English  3'oung  of  this  male  was  4.66; 
the  grade  of  his  homozygous  English  young  was  1.79.  The  corre- 
sponding figures  for  his  father  were  4.40  and  1.36  respectively. 

Three  other  sons  of  cf  5555  were  also  tested  by  matings  with  sub- 
stantially the  same  group  of  does,  although  tests  in  the  case  of  the  less 
promising  ones  were  terminated  sooner.  Male  6420  (table  36)  was  of 
slightly  lower  grade  than  his  father  and  was  found  to  be  probably 
inferior  to  him  and  so  was  soon  discarded.  His  heterozygous  English 
young  were  of  mean  grade  4.33. 


22  HEREDITY   IN   RABBITS,    RATS,    AND    MICE. 

j\Iale  6071  (table  37),  although  of  higher  grade,  gave  no  better 
results.  His  own  brother,  cf6072  (table  38),  born  in  the  same  litter 
and  graded  the  same,  did  much  better.  He  was  bred  very  extensively 
and  gave  a  record  very  similar  to  that  of  his  father,  6^6370,  who  was 
of  the  same  grade  but  had  half  a  generation  less  of  selected  ancestry. 
IVIale  6072  had  75  homozygous  Enghsh  young,  of  mean  grade  1.97 
(father's  record  1.79);  he  also  had  159  heterozygous  English  young 
of  mean  grade  4.63  (father's  record,  4.66).  Increase  in  the  grade  of 
the  young  with  increase  in  the  mother's  grade  is  very  clearly  shown 
among  the  young  of  this  sire.     (See  table  38.) 

The  next  male  tested  was  6964  (table  39)  a  son  of  cf  6071.  He  was 
discarded  after  a  set  of  matings  which  showed  him  probably  not 
better  than  his  uncle,  6072,  who  w^as  still  in  service.  He  had  21  homo- 
zygous j^oung  of  mean  grade  1.49  (his  uncle's  record  was  1.97)  and  29 
heterozygous  English  young  of  mean  grade  4.68  (his  uncle's  record 
being  4.63).  Next  was  tested  cr7699  (table  40),  son  of  6^6072,  who 
shared  with  his  half  brother,  cf  6370,  the  position  of  best  sire  so  far. 
All  were  of  the  same  grade,  5.  This  male  was  mated  with  all  available 
does  and  produced  354  recorded  young.  He  has  a  better  record  than 
any  sire  so  far  tested.  By  heterozygous  does  he  has  sired  75  homozy- 
gous young  of  mean  grade  2.31  and  149  heterozygous  young  of  mean 
grade  4.80. 

Another  male  of  the  same  grade  and  generation  as  the  foregoing, 
indeed  his  half-brother,  being  also  a  son  of  6072,  was  tested,  but 
appeared  not  to  be  superior  to  7699  and  so  was  soon  discarded.  This 
animal,  9532  (table  41),  sired  16  homozygous  English  young  of  mean 
grade  2.53  and  also  27  heterozygous  English  young  of  mean  grade  4.73. 

Three  sons  of  the  superior  male,  7699,  have  since  been  tested,  viz, 
9806,  1212,  and  534  (tables  42-44).  The  first  one  shows  no  probable 
superiority  over  his  father,  but  the  last  two  are  more  promising,  each 
having  produced  a  total  of  over  60  heterozygous  English  young  with  a 
mean  close  to  grade  5.  In  the  case  of  their  father  the  corresponding 
group  of  young  were  of  grade  4.80.  The  homozygous  young  produced 
by  their  father  were  of  mean  grade  2.31;  those  produced  by  the  sons 
were  of  mean  grade  2.87  and  2.95  respectively.  Accordingly,  as  regards 
both  heterozygous  and  homozygous  young,  the  sons  have  a  distinctly 
better  record.  This,  no  doubt,  was  due  in  part  to  the  fact  that  their 
mates  were  of  higher  grade  or  from  more  highly  selected  stock,  but  it 
was  not  wholly  due  to  this  cause,  for  their  half-brother  (9806,  table  42) 
did  not  show  the  superiority  which  they  showed,  even  when  mated 
with  females  of  high  grade  and  advanced  generations.  Hence  we  must 
conclude  that  these  two  males,  1212  (table  43)  and  534  (table  44),  were 
genetically  superior  to  their  father. 

Table  45  shows  the  grade  distribution  of  the  young  produced  by  a 
homozygous  English  male,  1173  (plate  3,  fig.  6),  when  mated  with  does 


INHERITANCE    OF    WIIITE-SPOTTINC    IX    RABBITS.  23 

of  the  three  categories  used  in  testing  heterozygous  Enghsh  males.  It 
will  be  noted  that  he  produced  only  English  young,  however  mated, 
conclusive  evidence  of  his  homozygous  dominant  character.  His  hetero- 
zygous English  young  were  of  mean  grade  4.77  and  4.84  by  homozygous 
English  and  self  does  respectively.  This  male  was  a  son  of  male  9532, 
table  41,  with  whose  genetic  character  his  own  was  very  similar,  judg- 
ing by  the  grade  of  their  heterozygous  English  young.  His  line  was 
not  continued. 

Table  46  enables  one  to  survey  at  a  glance  the  summarized  results 
of  this  entire  selection  experiment.  The  course  of  the  experiment  is 
followed  only  in  the  male  line,  because  only  in  the  case  of  the  males 
is  the  number  of  young  large  enough  to  show  beyond  question  the 
genetic  properties  of  the  individual.  From  the  beginning  of  the  experi- 
ment fluctuation  was  observed  in  the  grade  of  the  3'oung  produced,  and 
this  fluctuation  was  in  part  at  least  genotypic,  since  the  higher-grade 
mothers  have  given  higher-grade  young  in  matin gs  with  the  same 
male.  That  the  fluctuation  was  also  in  part  lihenotypic  is  shown  by 
a  comparison  of  the  records  made  by  different  males  of  the  same 
grade  when  mated  with  the  same  group  of  females  (tables  31-45). 

The  entire  selection  race  derives  its  English  character  from  cf2711. 
This  animal  was  a  heterozygote  deriving  the  English  character  in  a 
single  gamete  from  cf2545,  who  was  also  heterozygous.  Hence  the 
English  character  had  evidently  changed  in  transmission  from  father 
to  son,  a  sufficient  refutation  of  the  idea  of  unit-character  constancy. 
AVhether  the  change  resulted  from  a  directly  changed  unit-factor  (gene) 
or  from  the  introduction  of  one  or  more  modifying  factors  is  a  matter 
for  further  consideration. 

The  advances  made  in  the  male  line  seem  to  occur  as  five  successive 
steps  corresponding  roughly  with  generations  of  selected  ancestry 
(table  46).  The  first  advance  comes  with  the  selection  of  the  (single 
gamete)  male  2711,  founder  of  the  race;  the  next  in  the  selection  of 
his  grandson,  5555 ;  the  third  in  two  sons  of  5555,  viz,  6370  and  6072 ; 
the  fourth  occurs  in  the  selection  of  two  sons  of  6072,  viz,  7699  and 
9532;  the  fifth  is  seen  in  1212  and  534,  sons  of  7699.  The  direct  line 
of  advance  is  through  2711,  5555,  0072,  and  7699.  The  amount  of 
advance  at  each  step,  as  indicated  by  the  average  grade  of  the  young 
of  these  males,  is  shown  in  table  47.  The  rate  of  advance  has  evidently 
decreased  as  the  experiment  progressed. 

That  modification  of  the  English  pattern  resulted  imme<liately 
from  the  cross  with  self  individuals  of  an  unrelated  race  is  conclusi^•ely 
shown  in  table  30.  The  original  English  male,  2545,  i)roduced  by  Eng- 
lish mates  heterozygous  English  young  of  mean  grade  2.32;  by  self 
mates  he  produced  heterozygous  English  young  of  mean  gratle  2. SO, 
practically  half  a  grade  higher.  By  most  of  such  mates  the  young 
were  more  than  a  grade  in  advance  of  those  produced  by  English 


24  HEREDITY    IN   RABBITS,    RATS,    AND    MICE. 

mates.  Since  this  is  the  direct  effect  upon  the  Enghsh  character  of 
07ie  dose  of  the  self  race,  it  might  be  supposed  that  two  doses  would  have 
a  greater  effect,  so  that  if  it  were  possible  to  lift  the  English  character 
bodily  out  of  the  English  race  and  surround  it  with  the  complete  resid- 
ual heredity  of  the  self  race,  an  effect  perhaps  twice  as  great  as  that 
actually  observed  in  the  cross  might  be  expected.  Accordingly  an 
advance  of  between  2  and  2|-  grades  may  be  attributed  to  the  residual 
heredity  of  the  self  race.  Theoretically,  if  this  residuum  consisted  of 
a  number  of  independent  factors,  then  full  effect  would  be  secured 
upon  breeding  with  each  other  the  highest-grade  individuals  of  the 
cross-bred  race,  repeating  this  process  generation  after  generation 
until  each  factor  was  present  in  a  homozygous  state.  This  is  sub- 
stantially the  procedure  which  has  been  followed  in  the  5  full  genera- 
tions over  which  the  experunent  has  extended.  The  advance  realized 
amounts  to  about  2f  grades. 

Allowing  for  the  fact  that  one  of  our  arbitrary  "grades"  may  not 
have  the  same  genetic  value  as  another,  it  seems  probable  that  we 
have  secured  something  more  than  the  effect  of  the  residual  heredity 
of  the  self  race  employed  in  the  original  cross.  This  may  have  resulted 
either  from  a  process  of  elimination  from  the  heredity  complex  of 
factors  which  tended  to  lower  the  grade  of  the  English  character  or 
from  change  in  the  heredity  complex  by  some  other  process  than  addi- 
tion or  subtraction  of  factors — for  example,  by  change  in  factors. 

The  important  fact  which  this  experiment  demonstrates  is  the  same 
as  that  shown  in  the  selection  experiment  with  rats,  that  the  single 
characters  which  serve  to  identify  our  domestic  races  of  animals 
and  which  give  value  to  them,  even  though  they  conform  with  every 
criterion  of  unifactorial  Mendelian  heredity  in  transmission,  do  nev- 
ertheless vary  through  minute  gradations.  By  reason  of  the  fact  that 
the  residual  heredity  affects  such  characters,  a  cross  into  an  unrelated 
race  can  not  be  made,  except  with  the  possibility,  or  usually  with  the 
probabilitj^,  that  the  character  or  characters  in  ciuestion  will  be  thereby 
modified.  This  fact  was  formerly  expressed  in  the  statement  that 
"contamination"  of  unit-characters  frequently  follows  upon  cross- 
breeding— a  form  of  statement,  hov>'ever,  which  was  challenged  by 
those  who  maintained  that  the  gametes  v^^ere  "pure."  Subsequent 
investigation  has  shown  beyond  question  not  only  that  unit-characters 
are  frequently  greatly  modified  by  crosses,  but  also  that  they  can  be 
modified  by  selection  alone  unattended  by  crossing. 

Those  who  formerlj^  maintained  the  doctrine  of  gametic  purity  now 
shifted  their  ground,  and  while  admitting  that  unit-characters  might 
change,  insisted  that  single  factors  or  "genes"  could  not  change. 
This  is  the  doctrine  of  pure  genes  which  Morgan  has  made  so  familiar. 
This  doctrine  it  is  difficult  either  to  prove  or  disprove.  Pragmatically 
speaking,  it  is  of  small  consequence,  since  it  is  admitted  (1)  that  single 


Library 
N.  C,  State  College 


INHERITANCE    OF    WHITE-SPOTTING    IN    RABBITS.  25 

factors  do  sometimes  change,  leading  to  the  formation  of  nmhiple 
allelomorphs;  (2)  that  the  action  of  single  factors  is  not  limitetl  to  any 
particular  part  of  the  organism,  but  may  affect  parts  apparently 
unrelated;  (3)  that  the  total  number  of  factors  concerned  in  the  genesis 
of  even  the  simplest  organisms  must  be  very  great ;  and  (4)  that  in 
what  should  theoretically  be  "pure  lines"  (asexually  reproducing 
organisms,  Jennings)  genetic  changes  are  constantly  occurring.  Trag- 
matically,  then,  genetic  variability  by  minute  gradations  is  a  reality, 
precisely  as  Darwin  assumed  it  to  be,  and  this  fact  allows  races  to  be 
altered  steadily  and  permanently  by  selection,  either  natural  or 
artificial,  as  Darwin  also  assumed  was  the  case.  The  hypothesis 
that  stable  organic  forms  come  into  being  onl}'  suddenly,  by  aljrupt 
changes  from  preexisting  forms  and  not  by  gradual  modification — this 
hypothesis,  the  "mutation  theory"  as  commonly  understood,  is  not 
substantiated. 

RELATION  OF  DUTCH  TO  ENGLISH. 

It  remains  to  consider  the  genetic  relations  to  each  other  of  Dutch 
and  English  spotting.  Dutch,  as  we  have  seen,  behaves  as  a  reces- 
sive in  crosses  with  self-pigmented  races;  English,  on  the  other  hand, 
behaves  as  a  dominant.  Dutch  marking  appears  to  result  from  a 
simple  deficiency  of  pigmentation,  as  if  in  development  the  pigment 
supply  failed  at  an  extremity  or  at  an  embryological  point  of  finishing- 
off.  When  Dutch  marking  is  reduced  to  its  lowest  point  of  expression 
by  selection  or  crossing,  the  only  white  visible  is  found  at  the  tip  of 
the  nose,  or  on  the  toes  of  a  fore-foot,  or  as  a  spot  in  the  middle  of 
the  forehead.  English  spotting,  on  the  contrary,  appears  to  result 
from  some  positive  inhibiting  force,  some  agency  which  uses  up  the 
pigment-forming  materials  here  and  there  in  the  epidermis  and  con- 
verts them  into  an  end-product  not  colored  but  white.  That  English 
individuals  possess  all  the  agencies  necessary  for  full  pigment  formation 
is  shown  by  the  fact  that  English  parents  may  produce  fully  pigmented 
(self)  young  as  recessives,  which  then  produce  only  self  young  if  mated 
with  each  other. 

We  have  seen  that  there  occur  different  forms  of  Dutch  spotting, 
which  apparentlj^  behave  as  allelomorphs,  but  which  tend  to  become 
less  distinct,  one  from  another,  when  they  are  associated  in  the  same 
zygote.  It  would  seem  probable  that  they  represent  quantitati\ely 
different  stages  of  reduction  in  the  amount  of  some  substance  carried 
in  the  germ-cell.  But  undoubtedly  this  substance,  whatever  it  is.  is 
located  in  the  chromatin,  since  the  defect  is  transmitted  e(iually 
through  egg  and  sperm.  There  are  also  cjualitative  differences  among 
the  different  forms  of  Dutch,  as  for  example  between  'Mark"  and 
"tan"  Dutch,  in  one  of  which  the  white  collar  is  more  in  evidence, 
while  in  the  other  it  is  the  head  markings  that  are  more  in  evidence. 
Probably,  then,  the  different  forms  of  Dutch  are  variants  of  a  single 


26 


HEREDITY    IN   RABBITS,    RATS,    AND    MICE. 


Dutch  young. 

English  young. 

Grade. 

No. 

Grade. 

No. 

0 
1 
2 
3 

3 

2 
3 

1 

If 
2 

2i 
2\ 
2f 
3 

2 

15 

1 

2 

1 
5 

Total 

1 

9 

20 

' ' locus,"  in  the  terminology  of  the  chromosome  theory.  But  the  physi- 
ological and  genetic  behavior  of  English  are  so  difTerent  that  it  would 
seem  improbable  that  they  are  variants  of  the  same  gene. 

Nevertheless,  when  English  is  crossed  wdth  Dutch,  the  two  appear  to 
be  either  allelomorphs  or  closely  linked,  as  the  following  results  show : 
Heterozygous  English  rabbits  of  grade  5  were  crossed  with  ''white" 
Dutch  of  grades  15  to  17.  Two  matings  were  also  made  of  the 
homozygous  EngHsh  buck,  1173  (table  33,  and  plate  3,  fig.  6),  whose 
English  character  was  of  equivalent  potency  with  that  of  heterozygotes 
of  grade  5.  These  matings  produced  26  Enghsh  and  9  non-English 
(Dutch)  young,  which  were  graded  (with  no  great  exactness)  as  follows : 

There  was  probably  no  real  discontinuity 
in  the  grouping  of  the  English  young,  but 
owing  to  the  rough  manner  of  grading  them 
the  numbers  heap  up  on  the  even  grades, 
fractional  grades  being  neglected.  The  Dutch 
young  are  similar  in  grade  to  the  young  pro- 
duced by  crossing  "white"  Dutch  with  self . 
(Compare  table  27.)  But  the  English  young 
are  much  whiter  than  heterozygotes  between 
English  and  self.  The  latter  are  4.75  to  5.00 
in  grade  when  produced  by  the  same  English  sires.  (Compare  table 
45.)  But  the  Enghsh-Dutch  heterozygotes  in  no  case  are  of  higher 
grade  than  grade  3  and  in  the  great  majority  of  cases  are  no  darker 
than  grade  2.  In  other  words,  thej^  are  of  about  the  same  grade  as 
English  homozygotes.  This  means  that  a  white  Dutch  gamete  has 
about  the  same  whitening  effect  on  English  as  another  gamete  of  Eng- 
lish would  have.  The  pattern  of  the  English-Dutch  heterozygote  is 
indistinguishable  either  quantitatively  or  qualitatively  from  that  of  a 
homozygous  English  animal.  The  Dutch  is  not  at  all  in  evidence  except 
as  a  whitening  influence  on  the  dominant  English.  Even  in  a  single 
dose  it  completely  counteracts  the  darkening  influence  introduced  into 
the  English  race  in  the  process  of  the  cross  with  self  and  the  subse- 
quent 5  generations  of  selection. 

Since  it  has  been  shown  that  both  white  Dutch  and  English  segre- 
gate from  self  in  monohybrid  fashion,  much  interest  attaches  to  the 
inquiry  whether  they  segregate  from  each  other  in  the  gametes  of 
the  Fi  individuals.  Two  methods  have  been  employed  to  test  this 
matter,  one  being  to  mate  Fi  individuals  inter  se,  the  other  to  back- 
cross  them  with  white  Dutch.  If  English  and  Dutch  are  allelomorphs 
(invariably  pass  into  different  gametes)  nothing  but  English  or  Dutch 
young  should  be  produced  by  either  mating.  If,  however,  English 
and  Dutch  are  not  allelomorphs,  then  a  certain  number  of  gametes 
should  be  formed  by  Fi  individuals  which  are  neither  English  nor 
Dutch,  but  which  are  self,  and  a  like  number  should  be  formed  which 


INHERITANCE    OF   WHITE-SPOTTING    IN    RABBITS. 


27 


carry  both  English  and  Dutch,  the  genetic  properties  of  wliich  woukl 
be  to  produce  a  very  white  (low-grade)  English.  These  two  new  kinds 
of  gametes  would  respectively  be  as  numerous  as  the  simjile  iMiglish 
and  simple  Dutch  gametes,  if  English  and  Dutch  are  inherited  inde- 
pendently (in  different  chromosomes,  for  example).  If  English  and 
Dutch  are  linked  in  inheritance  (are  borne  in  homologous  chromosomes 
but  not  at  the  same  locus),  these  two  classes  of  gametes  would  represent 
the  "cross-overs."  The  existence  of  the  gamete  which  bears  both 
English  and  Dutch  would  be  difficult  to  demonstrate,  since  one  char- 
acter is  dominant,  the  other  recessive;  but  the  existence  of  gametes 
bearing  neither  English  nor  Dutch  would  be  easy  to  detect,  either  in 
the  straight  F2  generation  or  in  the  back-cross  generation.  Such 
gametes  uniting  with  each  other  would  produce  self  individuals,  or 
uniting  with  a  Dutch  gamete  would  produce  a  Dutch  of  grade  3  or 
lower.  Now,  among  47  F2  young  neither  of  these  classes  of  individ- 
uals has  appeared.  The  11  non-English  Fo  young  are  all  Dutch  of 
grade  7  or  higher.  An  even  better  test  for  the  existence  of  gametes 
transmitting  neither  English  nor  Dutch  is  the  back-cross  with  Dutch; 
for  in  this  case  any  such  gamete  would  produce  one  and  the  same  kind 
of  zygote,  viz,  Dutch  of  grade  3  or  lower.  No  such  zygote  has  appeared 
in  a  total  of  88  Dutch  and  105  English  young  obtained  in  back-cross 
matings.  This  indicates  that  Enghsh  and  Dutch  are  either  allelo- 
morphs or  closely  linked.  The  grade  distribution  of  these  back-cross 
young  is  showTi  m  the  table  herewith. 

It  should  be  of  interest  to  compare  this 
distribution  with  that  obtained  in  the  orig- 
inal English-Dutch  cross  (page  26)  as  indi- 
cating whether  the  contrasted  characters 
have  been  contaminated  in  the  Fi  zygote. 
The  English  back-cross  young  are  of  lower 
grade  (whiter)  than  the  Fi  Enghsh.  The 
respective  means  are  1.36  and  2.25. 

This  whitening  of  the  English  is  the  result 
of  contamination  from  white  Dutch  in  the 
Fi  zj^gote.  No  English  individual  produced 
in  the  back-cross  is  darker  than  the  English 
produced  in  Fi.  In  both  cases  the  darkest 
Enghsh  are  of  grade  3.  This  indicates  ab-  ' 
sence  of  gametes  transmitting  neither  English  nor  Dutch,  for  any 
such  gametes  uniting  with  an  English  gamete  should  prothice  lOnglish 
darker  than  gxade  3.  The  Fi  Dutch  (page  26)  were  self-white  Dutch 
heterozygotes.  (Compare  table  27.)  None  was  darker  than  grade  3. 
In  this  back-cross,  any  self  gamete  (neither  Dutch  nor  English)  which 
might  arise  by  cross-over  should  produce  young  ecjually  dark,  grade 
3  or  lower.     But  the  lowest-grade  Dutch  recorded  are  of  grade  0; 


Dutch 

young. 

English 

young. 

Grade. 

No. 

Grade. 

No. 

6 

2 

1 

52 

7 

1 

2 

25 

8 

1 

3 

2 

9 

4 

0 

26 

10 

6 

1        11 

4 

12 

13 

13 

1 

14 

15 

15 

14 

16 

22 

17 

3 

? 

2 

Total 

88 

105 

28  HEREDITY    IN   RABBITS,    RATS,    AND    MICE. 

hence  there  is  no  evidence  that  cross-over  gametes  are  produced  as 
often  as  once  in  192  times.  It,  therefore  appears  that  Enghsh  and 
Dutch  are  either  very  closely  coupled  or  are  variants  of  the  same 
locus.  As  regards  the  effect  of  the  English  cross  on  the  grade  of  the 
Dutch  character,  this  is  indicated  in  the  grade  of  the  back-cross  Dutch 
young,  which  range  in  grade  from  6  to  17,  average  13.88.  Similarly 
produced  back-cross  Dutch  obtained  from  Fi  (white  X  self)  X  white 
(table  17)  range  from  13  to  17  and  are  of  mean  grade  15.15.  This 
indicates  that  the  English  cross  has  darkened  white  Dutch  even  more 
than  self  did  in  a  similar  cross  (table  27).  The  superior  darkening 
effects  of  English  over  the  self  used  in  table  27  may  be  attributed  to 
the  more  highly  selected  character  (for  darkness)  of  the  English  used 
in  the  cross  and  to  the  less  highly  selected  character  (for  whiteness) 
of  the  Dutch  used  in  the  same  cross,  but  the  difference  is  not  great. 

At  any  rate,  from  the  grade  distribution  of  the  young  produced  in 
the  back-cross  of  Fi  (English  X  white  Dutch)  X  white  Dutch,  it  is 
clear  (1)  that  English  and  Dutch  behave  like  allelomorphs,  or  closely 
linked  factors,  since  in  nearly  200  cases  studied  no  cross-over  is 
observed,  i.e.,  no  gamete  transmitting  neither  English  nor  Dutch; 
(2)  that  the  segregated  English  and  Dutch  borne  by  the  gametes  of 
Fi  individuals  are  mutually  modified,  the  English  (previously  selected 
for  darkness)  being  made  lighter,  and  the  Dutch  (previously  selected 
for  whiteness)  being  made  darker.  In  these  mutual  modifications  we 
are  dealing  probably  for  the  most  part  with  residual  heredity,  but 
it  is  possible  that  quantitative  variation  of  the  English  and  the  Dutch 
genes  is  in  part  responsible,  yet  such  an  interpretation  is  not  favored 
by  the  results  obtained  from  the  critical  experiments  with  hooded  rats, 
which  point  strongly  to  changed  residual  heredity  as  the  correct 
explanation  of  changed  phenotypes,  when  only  a  single  Aiendelizing 
character  can  be  observed. 


III.  OBSERVATIONS  ON  THE  OCCURRENCE  OF  LINKAGE 

IN  RATS  AND  MICE. 

In  publication  241  of  the  Carnegie  Institution  evidence  was  pre- 
sented  showing  that  the  red-eyed  yellow  and  pink-eyed  yellow  varia- 
tions of  the  common  rat  {Mus  norvegicus)  are  due  to  genes  which  are 
linked  with  each  other.  Upon  crossing  with  each  other  the  two  yellow 
variations,  which  visibly  differ  in  eye-color  only,  young  are  obtained 
which  differ  from  both  parent  races  in  coat-color  as  well  as  in  eye- 
color.  These  young  are  black-coated  or  gray-coated  and  have  black 
eyes.  This  result  shows  clearly  that  the  two  variations,  which  are 
both  recessive  in  genetic  behavior,  are  due  to  independent  genes. 

In  the  F2  generation  the  two  yellow  varieties  were  recovered,  each 
wnth  its  distinctive  eye-color,  and  certain  individuals,  which  visibly 
were  pink-eyed  yellows,  were  found  from  breeding  tests  to  carry  the 
genes  for  red-ej'ed  as  well  as  for  pink-ej'ed  yellow.  These  double 
recessives  obviously  had  arisen  by  the  process  known  as  "crossing- 
over,"  in  which  genes,  although  introduced  in  a  cross  by  different 
parents,  yet  later  emerge  together  in  the  same  gamete  formed  by  an 
FjL  individual.  It  is  supposed  that  genes  which  behave  in  this  way  lie 
in  homologous  chromosomes  and  that  when  crossing-over  occurs  a 
gene  (A)  leaves  the  chromosome  in  which  it  originally  lay  and  crosses 
over  into  the  homologous  chromosome  in  which  the  other  gene  (B) 
lay.  Thus  both  A  and  B  come  to  lie  in  the  same  chromosome  and 
at  gametogenesis  pass  into  the  same  gamete.  From  an  examina- 
tion of  the  proportion  of  the  double  recessive  yellow  individuals 
found  among  the  F2  yellows,  it  w^as  concluded  that  cross-over  gametes 
(those  which  carry  genes  for  both  yellow  variations  or  for  neither) 
represent  about  18.5  per  cent  of  all  the  gametes  formed  b}'  Fi  individ- 
uals. If  no  linkage  occurred,  such  gametes  would  form  50  per  cent  of 
the  total. 

To  test  more  fully  the  strength  of  the  linkage  between  these  two 
genes  and  to  find  out  whether  this  linkage  has  the  same  strength  in 
both  sexes,  further  experiments  have  been  undertaken.  A  race  of 
homozygous  double  recessives  (genetically  both  pink-eyed  and  red- 
eyed)  was  built  up  from  the  F2  cross-over  individuals  and  with  this 
race  Fi  individuals  were  crossed.  If  we  designate  by  r  the  gene  for 
red-eyed  yellow  and  by  p  the  gene  for  pink-eyed  yellow,  an  Fi  individual 
might  be  expected  to  form  gametes  of  the  four  sorts  l^R,  Pr,  pR  and 
pr.  Of  these  4  combinations,  Pr  and  pR  would  corrcsjiond  with 
those  furnished  by  the  parent  races,  rcd-eyetl  yellow  and  pink-eyed 
yellow  respectively;  but  the  other  two,  PR  and  pr,  would  be  new  and 

29 


30  HEREDITY    IN   RABBITS,    RATS,    AND    MICE. 

could  therefore  arise  only  by  crossing-over.  A  test  mating  of  an  Fi 
individual  with  a  double  recessive  would  give  zygotes  as  follows,  it 
being  understood  that  the  double  recessive  individual  produces  only 
one  type  of  gamete,  viz,  pr.  Non-cross-over  gametes,  Pr  and  pR, 
would  give  zygotes  Pprr  and  ppRr,  visibly  red-eyed  yellows  and 
pink-eyed  yellows  respectively.  Cross-over  gametes,  PR  and  pr, 
would  give  zygotes  PpRr  and  pprr,  visibly  dark-eyed  (black  or  gray) 
and  pink-eyed  yellow  respectively.  The  pink-eyed  yellows  could  not 
be  distinguished  readily  from  yellows  arising  from  non-cross-over 
gametes,  but  the  dark-eyed  young  could  be  distinguished  immediately 
at  birth  from  all  other  classes.  Since  theoretically  they  would  con- 
stitute half  the  total  cross-overs,  it  is  evident  that  the  simplest  way  of 
estimating  with  accuracy  the  proportion  of  cross-over  gametes  is  to 
double  the  number  of  dark-eyed  young  observed  in  new-born  litters. 
This  number  divided  by  the  total  number  of  young  would  give  the 
percentage  of  cross-over  gametes. 

Following  this  procedure,  we  have  reared  from  matings  of  Fi  indi- 
viduals with  double  recessives  a  total  of  1,714  young,  of  which  174 
were  dark-eyed.  Doubling  the  number  174,  we  have  348  as  the  prob- 
able number  of  cross-over  gametes  among  the  1,714  Fi  gametes  which 
entered  into  the  production  of  these  young.  This  is  a  percentage  of 
20.3,  a  little  higher  than  the  calculation  18.5  of  publication  241,  based 
on  a  study  of  a  much  smaller  F2  population.  The  difference  between 
this  figure,  20.3  and  50,  the  percentage  expected  where  no  linkage 
occurs,  would  be  a  measure  of  the  strength  of  the  repulsion  shown 
between  the  genes  for  red-eyed  yellow  and  for  pink-eyed  yellow 
respectively,  when  they  enter  a  cross  in  different  gametes — that  is, 
each  through  a  different  parent,  the  condition  realized  in  this  cross. 

But,  on  the  chromosome  theory,  an  attraction  or  "coupling"  equal 
in  strength  to  this  repulsion  should  occur  between  the  same  two 
genes  when  they  enter  a  cross  together.  Entering  together,  they 
should  tend  to  hold  together,  because  they  would  lie  in  the  same  mem- 
ber of  a  pair  of  chromosomes  and  so  could  pass  out  separately  only 
in  consequence  of  a  cross-over.  This  point,  repeatedly  verified  in  the 
case  of  other  organisms,  was  tested  for  rats  by  producing  Fi  individuals 
through  a  cross  of  double  recessive  yellow  (pprr)  with  a  pure  non- 
yellow  individual  (PPRR).  In  reality  Fi  zygotes  of  this  same  sort 
were  being  produced  in  considerable  numbers  in  the  matings  already 
described  to  test  the  strength  of  repulsion.  Such  were  the  174  dark- 
eyed  young  already  mentioned.  Each  resulted  from  the  union  of  a 
pr  gamete  with  a  PR  gamete,  the  relationship  which  would  give  the 
expected  coupling.  Accordingly  many  of  these  dark-eyed  young  were 
used  instead  of  Fi  parents  in  the  experiments  to  test  the  strength  of 
"coupling"  between  p  and  r.  In  these  experiments,  as  in  those  to 
test  the  strength  of  repulsion,  Fi  individuals  were  mated  with  double 


OCCURRENCE    OF   LINKAGE    IN    RATS   AND   MICE.  31 

recessives.  In  both  cases  the  Fi  individual  was  of  the  formula  PpRr, 
the  double  recessive  was  of  the  formula  pprr. 

The  only  difference  in  the  two  cases  was  that  in  one  case  the  Fi 
arose  from  a  union  of  Pr  with  pR,  and  in  the  other  from  a  union  of 
pr  with  PR.  But  the  importance  of  this  circumstance  is  seen  in  the 
different  results  obtained  in  the  two  cases.  In  one  case  (where  p  and 
r  enter  the  cross  separately)  10  per  cent  of  the  young  were  dark,  in 
the  other  case  (where  p  and  r  enter  the  cross  together)  more  than 
40  per  cent  of  the  young  were  dark.  The  exact  figures  for  the  repulsion 
series  have  already  been  given,  174  dark  young  in  a  total  of  1.714,  or 
10.1  per  cent  dark  young.  For  the  coupling  series  the  figures  are 
1,255  dark  young  in  a  total  of  3,032,  or  41.3  per  cent  dark  young. 
To  compare  the  strength  of  repulsion  with  the  strength  of  coupling  we 
may  estimate  the  percentage  of  cross-over  gametes  produced  in  each 
case.  Either  sort  of  Fi  individual  would  produce  4  kinds  of  gametes, 
PR,  Pr,  pR,  and  pr.  But  in  the  repulsion  series  PR  and  pr  would  arise 
from  crossing-over,  whereas  in  the  coupling  series  Pr  and  pR  would 
arise  from  crossing-over.  In  either  case  dark-eyed  individuals  would 
arise  only  from  the  same  type  of  Fi  gamete,  viz,  PR,  but  in  the  repul- 
sion series  this  would  be  a  cross-over  gamete,  whereas  in  the  coupling 
series  it  would  be  a  non-cross-over  gamete.  While  in  the  repulsion 
series  the  number  of  dark-eyed  young  would  measure  half  the  total 
number  of  cross-overs,  in  the  coupling  series  it  would  measure  half 
the  total  number  of  non-cross-overs.  Applying  these  criteria,  we  have 
found  in  the  repulsion  series,  as  already  stated,  that  the  number  of 
dark-eyed  young  being  174,  the  probable  number  of  cross-over  gametes 
is  twice  this,  or  348,  in  a  total  of  1,714,  which  is  20.3  per  cent. 

Turning  now  to  the  coupling  series,  we  find  that  the  total  number  of 
dark-eyed  young  is  1,255.  Doubling  this  we  have  2,510  as  the  probable 
number  of  non-cross-over  gametes.  Deducting  this  number  from 
3,032,  the  total  number  of  young,  we  have  522  as  the  probable  number 
of  cross-over  gametes,  which  is  17.2  per  cent.  This  we  may  compare 
with  the  20.3  estimated  for  the  repulsion  series,  and  the  earlier  esti- 
mate of  18.5  based  on  the  census  of  an  Fo  population  (publication  241). 
These  differences  are  not  large  enough  to  lead  us  to  think  that  there 
is  any  consistent  difference  between  the  strength  of  repulsion  and  the 
strength  of  coupling  between  the  same  two  genes.  The  chromosome 
theory  would  not  lead  us  to  expect  the  existence  of  any  such  difference. 
This  case  therefore  fully  accords  with  that  theory.  If  we  combine  the 
results  obtained  from  both  the  repulsion  and  the  coupling  series  we 
have  as  the  average  linkage  strength  (either  repulsion  or  coupling,  as 
the  case  may  be)  18.3  per  cent.  This  is  based  on  a  total  of  4,740 
young  produced  by  the  back-cross  of  Fi  with  the  double  recessive.  The 
figures  are  large  enough  to  have  significance  and  agree  remarkably 
well  (almost  too  well)  with  the  estimate  based  on  the  F;  population, 


32  HEREDITY    IN   RABBITS,    RATS,    AND    MICE. 

viz,  18.5  per  cent.     It  is  safe  to  conclude  that  the  linkage  strength  of 
red-eyed  yellow  with  pink-eyed  yellow  is  close  to  18  per  cent. 

We  may  pass  now  to  the  question  whether  the  linkage  strength  is 
the  same  in  spermatogenesis  as  in  oogenesis,  whether  it  is  the  same 
among  the  gametes  formed  by  Fi  males  as  in  those  formed  by  Fi 
females.    A  priori  we  might  well  expect  it  to  be  different  in  the  two 
cases,  since  in  Drosophila  crossing-over  has  been  found  to  occur  only 
in  females,  whereas  in  the  silkworm  it  has  been  found  to  occur  only 
in  males.    In  publication  241  the  fact  was  demonstrated  that  crossing- 
over  does  occur  in  both  sexes  of  the  rat,  but  we  were  at  that  time 
unable  to  state  what  its  relative  frequency  was  in  the  two  sexes.    Our 
back-cross  series  of  matings  give  data  for  such  a  determination.    (See 
table  48.)     It  will  be  observed  that  the  estimated  percentage  of  cross- 
over gametes  is  somewhat  higher  for  females  than  for  males  in  both 
the  repulsion  and  the  coupling  series  and  that  the  difference  is  greatest 
where  the  numbers  are   largest,  viz,  in  the  coupling  series.     This 
would  suggest  that  crossing  over  occurs  more  readily  in  oogenesis 
than  in  spermatogenesis,  but  I  doubt  very  much  whether  such  is  the 
case  when  all  other  conditions  are  the  same.    Summaries  made  for  the 
concluding  period  of  our  experimental  work,  when  conditions  had  been 
more  carefully  controlled  and  the  procedure  of  taking  the  records  had 
been  best  standardized,  show  no  appreciable  differences  in  the  case 
of  the  two  sexes.    For  this  period,  in  the  coupling  series,  Fi  females 
gave  187  dark  and  277  yellow  young,  or  19.3  per  cent  cross-over 
gametes.    Simultaneously,  Fi  males  of  similar  parentage  gave  133 
dark  and  197  yellow  young,  or  19.4  per  cent  cross-over  gametes.    In 
the  repulsion  series  only  Fi  males  were  at  this  time  being  used  to 
any  great  extent.    They  produced  29  dark  young  and  269  yellow 
young,  which  by  the  method  of  calculation  already  explained  indi- 
cates 19.4  per  cent  cross-over  gametes,  a  remarkably  close  agreement 
with  the  results  given  by  both  sexes  in  the  coupling  series  at  this 
same  period. 

Whether  external  conditions  have  any  influence  on  the  percentage 
of  cross-overs  we  are  unable  to  state,  but  this  seems  doubtful  in  the 
case  of  a  warm-blooded  animal  such  as  the  rat.  That  individual  or 
age  differences  may  occur  among  Fi  animals  affecting  the  percentage 
of  cross-overs  is  a  possibility  we  have  considered  carefully,  but  with 
only  negative  conclusions.  The  indicated  percentage  of  cross-overs 
varies  in  the  case  of  particular  Fi  males  from  0  to  44  per  cent,  but  this 
variation  appears  to  be  the  result  of  random  sampling  rather  than  of 
consistent  differences  in  genetic  behavior.  Several  males  showing 
extremely  high  or  extremely  low  percentage  of  cross-overs  were  trans- 
ferred to  new  breeding-pens  and  mated  with  other  double  recessive 
females.  Their  indicated  percentages  of  cross-over  gametes  before 
and  after  the  transfer  showed  no  consistency  with  each  other,  and 


OCCURRENCE    OF    LINKAGE    IX    HATS   AND    MICE.  33 

SO  we  are  forced  to  conclude  that  individual  differences  as  regards  the 
production  of  many  or  few  cross-overs  have  not  l)cen  shown  to  exist. 

In  publication  241  evidence  was  presented  indicating  that  all)inism 
in  rats  is  probably  due  to  a  gene  which  is  linked  with  the  genes  for  red- 
eyed  yellow  and  pink-eyed  yellow.  This  idea  is  now  fully  established 
and  we  are  able  to  give  provisional  estimates  of  the  linkage  strengths 
involved,  although  the  investigation  of  this  matter  is  still  incomplete. 

When  red-eyed  and  pink-eyed  rats  are  crossed  with  each  other,  or 
either  sort  is  crossed  with  an  all^ino,  the  Fi  young  produced  are  dark- 
eyed  and  dark-coated  (either  black  or  gray,  according  as  the  agouti 
factor  is  absent  or  present).  But  the  Fi  young  are  not  quite  as  dark 
in  color  as  wild  rats.  This  shows  that  all  three  variations  are  reces- 
sive and  complementary,  but  that  the  allelomorph  of  each  is  a  little 
less  effective  in  producing  pigment  when  in  heterozygous  form  than 
when  in  homozj-gous  form  (as  in  wild  rats,  or  in  Irish  or  in  hooded 
rats).  Fi  individuals  from  the  cross  of  albino  with  pink-eyed  yellow, 
when  bred  with  each  other,  produce  an  Fo  generation  of  three  apparent 
types,  viz,  (1)  dark,  in  eye  and  coat  color;  (2)  pink-eyed  yellow; 
(3)  albino.  If  no  linkage  occurred  we  should  expect  these  three 
classes  to  occur  in  the  ratio  9  :  3  :  4;  but,  as  was  pointed  out  in 
publication  241,  linkage  would  tend  to  equalize  the  numbers  of  pink- 
eyed  and  albino  j^oung,  and  such  a  tendency  has  been  recorded. 
Further,  if  no  linkage  occurs,  but  if  pink-eyed  yellow  and  albinism 
segregate  quite  independently  of  each  other,  then  half  the  albino 
gametes  formed  by  Fi  individuals  should  transmit  pink-eyed  yellow 
and  half  should  not  transmit  it;  conversely,  half  the  gametes  which 
transmit  pink-eyed  j-ellow  should  also  transmit  albinism  and  half 
should  not.  If  less  than  half  the  gametes  which  transmit  one  character 
transmit  the  other,  the  two  show  repulsion. 

To  test  the  matter,  45  F2  albinos  have  been  mated  with  homozygous 
pink-eyed  individuals.  Of  the  45  so  mated,  17  have  produced  both 
pink-eyed  and  dark-eyed  young,  one  has  produced  only  pink-eyed 
young,  and  27  have  produced  only  dark-eyed  young.  The  17  are 
evidently  heterozygous  for  pink-eye  as  well  as  homozygous  for  alliinism, 
their  formula  being  ccpP.  The  one  which  produced  only  pink-eyed 
young  is  probably  of  the  formula  ccpp.  The  27  which  produced  only 
dark-eyed  young  are  of  the  formula  ccPP. 

We  may  now  consider  what  was  the  nature  of  the  gametes  which 
produced  these  45  individuals.  A  gamete  which  furnished  both 
albinism  and  pink  may  be  called  a  cross-over  gamete;  one  which 
furnished  albinism  only  must  be  regarded  as  a  non-cross-over  gamete. 
The  27  albinos  which  did  not  transmit  pink-eye  evidently  arose  each 
from  the  union  of  two  non-cross-over  gametes.  This  accounts  for 
2  X  27  or  54  non-cross-over  gametes.  The  17  individuals  which 
were  heterozygous  for  pink  evidently  received  each  a  single  non-cross- 


34  HEREDITY   IN   RABBITS,    RATS,    AND    MICE. 

over  gamete.  This  makes  a  total  of  71  such  gametes.  Cross-over 
gametes  were  represented  singly  in  each  of  the  17  individuals  which 
were  heterozygous  for  pink  and  doubly  in  the  one  which  was  homo- 
zygous for  pink.  This  makes  a  total  of  19  cross-over  as  against  71 
non-cross-over  gametes,  which  is  21.1  per  cent  cross-overs.  This  is  an 
indicated  linkage  strength  a  little  less  close  than  that  between  pink- 
eyed  and  red-eyed  yellow,  in  which  case  the  percentage  of  cross-over 
gametes  was  estimated  at  18.3.  For  with  no-linkage  gi\'ing  50  per 
cent  cross-overs,  it  is  evident  that  the  linkage  strength  increases  as 
the  percentage  of  cross-overs  decreases  until  (when  cross-overs  cease) 
linkage  becomes  complete.  If  we  measure  the  strength  of  linkage  by 
the  difference  between  the  observed  percentage  of  cross-overs  and 
50  per  cent  (the  percentage  of  cross-overs  when  no  linkage  occurs), 
then  linkage  between  red-ej^ed  yellow  and  pink-eyed  yellow  is  31.7  and 
that  between  pink-eyed  yellow  and  albinism  is  28.9,  as  provisionally 
determined. 

The  linkage  between  red-ej^ed  yellow  and  albinism  is  much  stronger 
than  the  linkage  in  either  of  the  cases  just  discussed.  Tests  have 
been  made  for  the  presence  of  the  red-eyed  yellow  gene  in  160  F2 
albinos  and  for  the  presence  of  albinism  in  57  F2  red-eyed  yellows 
derived  from  the  cross  of  albino  with  red-eyed  yellow.  Only  a  single 
cross-over  has  been  detected,  and  even  that  is  not  beyond  question. 
One  of  the  Fo  albinos,  a  male,  when  mated  with  a  pure  red-eyed  yellow 
female,  sired  a  litter  of  young,  all  of  which  were  dark-eyed  except  one. 
This  one  proved  to  be  a  yellow  but  died,  as  did  the  father,  before 
additional  breeding  tests  could  be  applied.  If  this  yellow  individual 
was  really  sired  by  the  albino  male  (and  not  accidentally  introduced 
from  some  other  cage,  a  remote  possibility),  then  that  male  evidently 
carried  yellow  as  well  as  albinism  and  in  his  genesis  a  cross-over 
gamete  must  have  functioned.  Each  of  the  other  F2  albinos  and  the 
Fo  yellows  tested  manifestly  arose  from  the  union  of  gametes  neither 
of  which  transmitted  both  yellow  and  albinism,  since  as  mated  they 
produced  only  dark-eyed  young  (4  or  more  each).  On  these  assump- 
tions the  experiments  thus  far  show  that  only  one  gamete  out  of  434 
formed  by  Fi  parents  can  have  been  a  cross-over  gamete,  which 
apparently  gives  less  than  one  per  cent  of  cross -overs. 

The  experiments  are  being  continued  with  the  hope  of  finding  addi- 
tional cross-overs  and  of  thus  securing  a  double  recessive  race,  which 
will  make  possible  a  more  accurate  determination  of  the  linkage 
strength.  The  information  already  presented  shows  that  on  the 
chromosome  theory  the  genes  for  albinism  and  for  red-eyed  yellow  are 
extremely  close  to  each  other  in  the  same  chromosome  and  that  the 
gene  for  pink-eyed  yellow,  while  lying  in  this  same  chromosome,  is 
at  some  distance  from  the  genes  for  albinism  and  red-eyed  yellow. 


OCCURRENCE   OF   LINKAGE    IN    RATS   AND    MICE.  35 

In  mice  it  has  been  shown  by  Haldane  6/ a/,  that  the  genes  for  albin- 
ism and  for  pink-eye  are  probably  linked  with  each  other.  This  fact, 
interesting  in  itself,  is  made  doubly  so  by  the  consideration  that 
characters  apparently  identical  in  nature  with  these  two  are  also 
linked  with  each  other  in  rats.  Since  mouse  and  rat  are  species  grouped 
by  systematists  in  a  single  genus,  it  should  be  of  interest  to  compare 
their  genetic  constitution  as  fully  as  possible.  With  this  idea  in  mind 
we  had  already  undertaken  to  study  the  linkage  relations  of  all)iiiism 
and  pink-eye  in  mice  before  the  appearance  of  the  paper  by  Ilaldane 
et  aU  This  investigation  was  undertaken  by  Mr.  L.  C.  Dunn  while 
acting  as  my  assistant.  Upon  his  entering  military  service,  I  took 
over  the  experiments.  It  is  a  pleasure  to  acknowledge  Mr.  Dunn's 
important  part  in  the  work. 

We  began,  as  in  the  rat  experiments,  by  crossing  pink-eyed  with 
albino  individuals.  Dark-eyed  Fi  young  were  produced  exactly  as 
in  rats.  These  bred  with  each  other  produced  an  F2  generation  of 
dark-eyed  young,  pink-eyed  young,  and  albinos,  in  a  9  :  3  :  4  ratio 
manifestly  modified  by  linkage.  Pink-eyed  F2  individuals  were  tested 
for  the  presence  of  albinism  and  albino  F2  individuals  were  tested  for 
the  presence  of  the  pink-eye  gene  as  a  first  step  toward  the  production 
of  a  race  of  double  recessives  needed  to  ascertain  the  proportion  of 
cross-over  gametes  formed  by  Fi  individuals.  The  simplest  way  of 
making  the  tests  was  found  to  be  the  mating  of  F2  albinos  with  F2 
pink-eyed  individuals.  This  afforded  simultaneously  a  test  of  both 
parents.  For  if  the  pink-eyed  parent  carried  albinism,  50  per  cent  of 
the  young  would  be  albinos,  otherwise  none  would  be  albinos.  But 
jf  the  albino  carried  the  pink-eye  gene,  50  per  cent  of  the  young  pro- 
duced would  be  pink-eyed.  If  both  these  contingencies  were  realized 
in  the  mating,  25  per  cent  of  the  young  would  be  pink-eyed  and  25  per 
cent  albinos.  All  other  young,  as  in  a  cross  of  pure  pink-eyed  with 
pure  albinos,  would  be  dark-ej^ed. 

If  no  linkage  occurred  between  pink-ej'e  and  albinism,  it  would  be 
expected  that  half  the  F^  pink-eyed  individuals  would  carry  albinism, 
and  also  that  half  the  albinos  would  carry  pink-eye.  Any  smaller 
proportions  than  these  of  pink-eyed  carrying  albinism  or  of  albinos 
carrying  pink-eye,  among  the  F2  individuals,  would  indicate  linkage. 

Linkage  is  very  clearly  shown  by  the  tests  made.  Among  03  F? 
pink-eyed  which  were  tested,  18  produced  each  one  or  more  albino 
young  in  litters  otherwise  dark-eyed,  while  45  produced  no  albinos 
but  only  dark-eyed  young.  In  the  genesis  of  the  18  parents  men- 
tioned, it  is  evident  that  18  cross-over  gametes  had  united  with  18 

^  In  fact,  I  have  not  yet  had  access  to  the  paper  by  Haldane  et  al.  but  know  it  only  as  cited 
by  others.  Our  copy  of  the  journal  in  which  it  appeared  is  probably  at  the  bottom  of  the  ocean 
and  we  have  been  unable  as  yet  to  replace  it. 


36  HEREDITY   IN   RABBITS,    RATS,    AND    MICE. 

non-cross-overs.  But  in  the  production  of  the  45,  only  non-cross-over 
gametes  had  functioned.  The  total  gametes  involved  accordingly  are 
18  cross-overs  and  18  +  (2  X  45)  =  108  non-cross-over  gametes; 
total  126.  As  18  is  14.28  per  cent  of  126,  the  indicated  percentage  of 
cross-overs  is  14.28  per  cent. 

Among  75  F2  albinos  which  were  tested,  20  produced  pink-eyed 
young  (as  well  as  dark-eyed  ones),  while  the  remaining  55  produced 
only  dark-eyed  young.  Reasoning  as  before,  there  were  evidently 
in\'olved  in  this  case  20  cross-over  gametes  and  20  +  (2  X  55)  =  130 
non-cross-overs,  total  150.  But  20  is  13^  per  cent  of  150;  hence  the 
indicated  percentage  of  cross-overs  is  13^. 

Combining  the  tests  of  F2  pink-eyed  and  of  F2  albinos,  we  have  in 
tests  involving  276  Fi  gametes  an  indicated  percentage  of  13.76 
cross-overs. 

The  pink-eyed  parents,  which  in  the  course  of  these  tests  had  been 
found  to  carry  albinism,  were  now  mated  with  each  other,  and  the 
albino  young  which  they  produced  when  so  mated  w^ere  used  in 
building  up  a  race  of  double  recessives,  for  all  albinos  so  produced 
must  of  necessity  be  homozygous  for  pink-eye.  These  double  reces- 
sives were  next  mated  with  Fi  dark-eyed  animals  obtained  by  the 
original  cross  of  pink-eyed  with  albino,  or  with  dark-eyed  individuals 
of  similar  genetic  constitution  which  had  resulted  from  the  test 
matings.  The  interpretation  of  the  results  obtained  from  these  back- 
cross  matings  is  the  same  as  that  given  by  similar  matings  in  the  case 
of  rats.  The  Fi  parent  would  form  gametes  of  the  four  sorts  CP,  cP, 
Cp,  and  cp,  of  which  cP  and  Cp  would  represent  the  original  com- 
binations found  in  pure  albinos  and  pure  pink-eyed  respectively,  and 
so  would  be  non-cross-overs,  but  CP  and  cp  would  arise  only  by  crossing- 
over.  Of  the  four  types  of  gamete,  CP  alone  would  produce  a  dark- 
eyed  zygote,  if  mated  with  a  double  recessive,  cp.  But  this  is  one  of 
the  two  cross-over  types.  Hence  the  number  of  dark-eyed  young 
produced  in  mating  Fi  animals  with  double  recessives  should  indicate 
half  the  total  percentage  of  cross-overs.  By  matings  of  the  sort  just 
described,  3,142  young  have  been  produced,  of  which  222  were  dark- 
eyed.  Doubling  this  number,  we  have  444  as  the  probable  number  of 
cross-over  gametes  in  3,142  gametes  produced  by  the  Fi  parents,  an 
indicated  percentage  of  14.13.  This  agrees  very  well  indeed  with  the 
13.76  per  cent  indicated  by  the  test-matin gs  of  F2  pink-eyed  and 
albinos.  It  seems  safe  to  assume,  therefore,  that  the  average  cross- 
over percentage  is  close  to  14  per  cent.  For  the  corresponding  char- 
acters in  rats,  the  indicated  percentage  of  cross-overs  is  considerably 
higher,  viz,  21.1,  but  it  should  be  borne  in  mind  that  the  estimate  is 
based  on  a  much  smaller  series  of  observations  in  the  case  of  rats  and 
that  further  observations  may  alter  it  materially. 


TABLES. 


37 


TABLES. 

Table  1. — Classification  of  generation  17,  plus-sclcclion  series. 


Grade 

of 
parents. 

Grade  of  ofTspring. 

Totals. 

Means. 

31 

4 

4i 

4^ 

H 

5 

51 

5i 

5J 
2 

4t 

4^ 
4j 

H 

4i 
5 

51 
5i 
5| 

2 

1 

1 
5 
1 
9 
3 
1 

2 
2 

1 
4 
7 
12 
8 
5 
2 
3 

1 

3 

5 

11 

7 

26 

10 

10 

13 

8 

2 

3 

4 

7 

11 

17 

13 

12 

6 

1 

1 

1 
4 
6 

13 
5 

10 
2 
2 

1 

4 
2 
8 
1 
10 
5 
1 
1 

1 
1 
4 
6 
3 
4 
3 

1 
2 

5 
3 

5 
9 

12 
31 
38 
88 
43 
65 
39 
16 
5 

3.95 
4.22 
4.44 
4.47 
4.49 
4.46 
4  48 
4.56 
4.61 
4.30 
4.55 

4.81 

23 

46 

96 

75 

44 

32 

22 

11 

o 

351 

4.48 

Table  2. — Classification  of  generation  IS,  plus-selection  scries. 


Grade 

of 
parents. 

Grade  of  offspring. 

Totals. 

Means. 

31 

3i 

4 

4i 

4^ 

41 

5 

5i 

5^ 

4i 
4^ 
4^ 
4i" 
4f 
4|- 
5 

51 
5i 
5i 

3 

4 

5 

2 

5 

7 
o 

1 

5 

9 
3 

10 
3 
8 

11 
4 
2 

3 

9 

8 

22 

13 

14 

25 

11 

7 

2 

1 

9 

10 

27 

7 

5 

16 

14 

5 
3 

3 

7 

12 

6 

2 

12 
2 
3 
2 

2 
2 
5 

4 
8 
1 
9 
7 
4 

1 

1 
5 
6 

3 
2 

5 
2 

1 
1 
3 

1 
1 

2 

19 
32 
35 
86 
48 
35 
84 
43 
27 
11 

4.10 
3.34 
4.57 
4.45 
4.64 
4.21 
4.43 
4.50 
4.61 
4.82 

4.S0 

3 

26 

55 

114 

97 

49 

42 

25 

9 

420 

4.46 

Table  3. — Classification  of  generation  19,  plus-selection  series. 


Grade 

of 
parents. 

Grade  of  offspring. 

Totals 

Means. 

H 

3f 

4 

4i 

4i- 

4| 

5 

5i 

5i 

4i 

4i 

4i 

4^- 
4f 
41 
5 

5i 

3 

8 

4 
1 
3 

2 
15 

3 
6 

1 

4 
25 
4 
4 
15 
6 
4 
3 

1 

23 

4 

5 

20 

12 

7 

6 

9 
3 
2 
8 
10 
3 
3 

7 
2 
1 
8 
8 
3 
2 

4 

2 
3 

8 

1 

1 

1 
2 

0 
95 
13 
17 
65 
47 
22 
14 

4.21 
4.34 
4.56 
4.50 
4.49 
4.76 
4.47 
4.53 

4.66 

3 

16 

27 

65 

78 

38 

31 

18 

4 

280 

4.49 

38 


HEREDITY   IN   RABBITS,    RATS,   AND   MICE. 


Table  4. — ClassificaHori  of  generation  20,  plus-selection  series. 


Grade 

of 
parents. 

Grade  of  offspring. 

Totals. 

Means. 

3f 

4 

4i 

4^ 

4f 

5 

5| 

5i 

5| 

4^ 

4| 

^ 

4| 
4f 
4J 
5 

51 
5^ 

1 

2 

1 
2 

1 

3 
5 
9 

1 
1 
1 

1 

3 
5 
3 

4 
3 
3 

7 

2 
3 
1 
3 
1 

3 

1 
2 
1 
3 
2 
2 

1 

1 
3 

2 

2 

1 

2 

9 

15 

19 

7 

14 

13 

9 

3 

3 

4.22 
4.50 
4.42 
4.57 
4.77 
5.02 
4.61 
4.89 
4.75 

4.66 

1 

6 

21 

28 

13 

11 

7 

3 

2 

92 

4.61 

Table  5. — Classification  of  generation  18,  minus-selection  series. 


Grade  of 
parents. 

Grade  of  offspring  (minus). 

Totals. 

Means. 

2i 

2h 

2| 

3 

3i 

H 

3f 

4 

-2| 

-2i 

-3 

-3i 

-3i 

-3f 

-H 

-31 
-3t 

3 
3 
2 

26 
5 

18 
5 

1 

4 

29 
28 
37 
19 
15 
12 
3 
9 

2 

9 

9 

13 

8 
8 
1 

4 

1 

1 
4 
2 
5 
8 
6 
2 
5 
1 

4 
1 
1 
3 
6 
1 

1 

1 
2 

1 
1 

61 

49 
68 
47 
34 
28 

9 
29 

5 

-2.63 
-2.78 
-2.72 
-2.94 
-2.97 
-3.00 
-3.14 
—3.03 
-3.45 

-3.09 

8 

59 

152 

55 

34 

16 

4 

2 

330 

-2.84 

Table  6. — Classification  of  generation  19,  minus-selection  series. 


Grade  of 
parents. 

Grade  of  offspring  (minus). 

Totals. 

Means. 

2i 

2| 

2f 

3 

3i 

3J 

4 

-2f 
-2| 
-3 

-3i 

-H 

-31 
-31 

3 

1 
5 
4 
3 

2 

2 
11 
16 
11 

7 
2 
1 

2 
3 

7 

15 

2 

7 

2 

1 
3 
7 
2 
2 

1 
1 

1 
2 
1 

2 

1 

5 

21 

36 

37 

11 

13 

3 

3 

1 

-2.80 
-2.79 
-2.85 
-2.95 
-2.89 
-3.08 

-3.10 

3 

15 

50 

38 

17 

6 

1 

130 

-2.89 

TABLES. 


39 


Table  7. — Classification  of  generation  20,  rninus-seUction  aeritt. 


Grade  of 
parents. 

Grade  of  offspring  (minus). 

Totals. 

Means. 

2 

1 

ol 

-4 

2i 

2J 

3 

3i 

3J 

-2i 
-2| 

-n 

-3 
-3i 

1 

1 

8 

6 
1 

9 
2 

21 
2 

5 

3 

1 
6 
4 

1 
6 

1 

21 
5 

40 
7 
6 

-2.67 
-2.80 
-2.81 
-2.86 
-2.S7 

-2.S1 

1 

2 

1.5 

39 

14 

1 

79 

-2.78 

Table  8. — Classification  of  generation  21,  minus-selection  series. 


Grade  of 
parents. 

Grade  of  offspring  (minus). 

Totals. 

Means. 

2 

2\ 

oi 

2f 

3 

3i 

3^ 

-2| 
-2f 

-2} 

-3 

-31 

1 

2 
1 

7 
4 
2 

1 

2 
1 

1 
1 
2 

3 

1 
1 

2 
3 

14 

7 

3 

1 

10 

-2.61 
-2.G1 

-3.11 

-2.58 

1 

3 

14 

7 

3 

O 

5 

35 

-2.74 

Table  9. — Classification  of  extracted  hooded  third  Fj  young  produced  by  a 
cross  of  plus-selected  un.th  wild  rats. 


2d  F-z  grand-parent. 

2 

n 

ol 

-2 

o3 

-4 

3 

31 

3i 

Totals. 

Means. 

a 

9  208,  +2l 

9  9922,  +3 

2 

1 

o 

0 

1 

1 
1 

1 
2 

6 

1 
9 
9 

.... 

cf63,  +  3i 

Weighted  mean,  3.22. 

o 

1 

2 

2 

1 

2 

9 

19 

3.04 

.64 

Note.  Standard  deviation  oi  first  F2  was  0.73,  that  of  second  Fj  was  0.50  (tables 
141  and  145,  Publication  No.  241). 


Table  10. — Classification  of  extracted  hooded  first  F2  young  from  a  cross 

minus-selected  with  ivild  rats. 


of 


Hooded  grand-parent. 

n. 

2  If 

n 

li 

1 

4 
4 

13 
4 

26 

f 
1 

1 

2 

7 
1 

10 

_i 
4 

1 
1 

2 

0 

8 
1 

7 
3 

19 

+  i 

i 

2 

1 

3 

f 

1 

1 

1 
4 
4 

8 

li 
1 

1 

1 
1 

2 
3 

•• 
t 

n 

1 

1 

1 

3 

2 
2 
1 
3 

2i 

2i 
2 

23: 
I 

>i 

H 

29 

I    16 

5 

53 

18 

1  121 

c 

09 
2 

a 
1.26 

9  20,331,  -2f.  gen.  15^ 
9  20,482,  -2igen.  1.5f 
920,359,  -2i  gen.  16. 
9  20,327,  -2f,  gen.  15| 
9  20,480,  -2f,  gen.  lof 

2 

1 
3.. 
2   2 
2   7 
.    6 

716 

1 

6 

7 

1 
1 

+  .46 
-.41 

-.65 
-.49 

— 

0 

1 

Total 

2 

-.38 

40 


HEREDITY   IN   RABBITS,    RATS,    AND   MICE. 


Table  11. — Classification  of  extracted  hooded  second  Ft  young  from  the  cross  of 

minus-selected  with  wild  rats. 


From 
original 
hooded. 

1st  Fa  grand-parent. 

If 

1 

1 

li 

a 

1 

1 
1 

3 

4 

1 
2 

1 

1 

1 
4 

0 

2 
4 

_ 
7 

+i 

1 

2 

2 
2 
3 

1 

8 

3 

4 

1 
1 

9 

1 
1 

2 
1 
1 

5 

4 

1 

5 

1 

3 
2 
6 

If 

1 
1 

2 

4 

2 

1 

2 
2 

5 

2i 

2| 

2f 

m 

o 

Means. 

O 

9  20,480.. 
9  20,327.. 
9  20,327.. 
9  20,480.. 

9  1G9S,  -If 

91715,  -If 

91563,  -1^ 

9  944,    +1^ 

1 

1 

1 
1 

14 

17 

9 

9 

1.11 
.80 
.67 

1.64 

Weighted  mean,  —1.11 

1 

1 

2 

49 

1.01 

.92 

Table  12.' — Classification  of  extracted  hooded  third  Fi  young  from  a  cross  of 

7ni?ius-selected  with  wild  rats. 


From  1st  F2. 

From  2d  F2. 

1 

li 

.  . 

1 

1^ 

1 

2 
2 

If 

3 

1 
1 
2 

1 

2 

2 
2 

1 

1 

2J 

2~ 

2f 

3 

3| 

H 

0 

a 

QJ 

0 

9944,     -li... 
9944,    +lh-.- 
9  944,    +li.... 
91563,  -li... 
91698,  -If...  . 
91563,  -li... 

9  1924,  +2 

3 

1 

2 
2 

1 

9 
3 

1 

1 

8 
3 
5 

1 

2 

8 
6 
2 

1 
4 

4 
3 
3 
2 

0 

1 

4 
5 

41 
23 
13 

7 

6 

14 

2.52 
2.51 
2.68 
2.25 
1.79 
3.05 

9  1925,  +^ 

cf  1926,  +lh 

9  2008,  +^ 

1 
1 

cf  2048,  +lf 

c^2068,  +3 

Weighted  mean,  +1.62.  ..  . 

6 

1 

5 

8 

6 

5 

14 

19 

21 

14 

104 

2.55 

.66 

Table  13. — Grade  distribution  of  Fi  young  sired  by  the  standard-bred  Dutch  <fS037 
{grade  7)  mated  with  does  transmitting  the  self  {unspotted)  condition. 

Mothers  designated  (E)  were  English  marked  and  transmitted  the  self  condition  in  only 
half  of  their  gametes.  The  English  young  of  such  mothers  are  omitted  from  this  table.  Mothers 
designated  (H)  were  Himalayan  albinos. 


Parents. 

Grades  of 
young. 

Totals. 

Mean 
grade. 

0 

1 

2 

3 

(^3037(72)  X   9  2651(E) 

X  9  2688(E) 

X  9  2687(H) 

X  9  2830(H) 

X  9  2835(H) 

3 

1 
1 
4 
5 
4 

1 

1 
1 

1 

2 

1 
3 
5 

9 

4 

I2.I7 
[    .95 

Totals 

3 

15 

3 

3 

24 

1.25 

Table  14. — Grade  distribution  of  the  back-cross  young  produced  by  Fi  does 
{table  IS)  mated  with  the  standard-bred  Dutch  buck  cfSOSS  {grade  9). 


Parents. 

Grades  of  young. 

Totals. 

Mean. 

1 

2 

3 

4 

5 

6 

7 

c?3036(92)  X  9  5001(1) 

X  9  5032(1) 

X  9  5003(3) 

2 

4 

2 
1 

1 

1 

4 

3 

2 

9 
5 
6 

Totals 

2 

4 

3 

1 

5 

5 

20 

4.00 

■  The  number  in  parentheses  indicates  the  grade  (pi.  1)  of  the  animal. 


TABLES. 
Table  15. — Grade  distribution  of  F2  young  from  the  cross  indicated  in  table  IS 


•n 


Grades  of  young. 

Totals. 

Mean 
grade. 

2.45 
1.34 

J.  arents. 

0 

7 

1 

1 

3 
3 

1 

14 

4 

2 

2 

2 
3 
5 
3 

3 

3 

1 
5 
1 
3 
2 

4 

1 

1 
2 

5 
2 

1 

cf 5002(3)  X  9  5001(1).. 

X   9  5032(1).. 

X   9  5003(3).. 
c? 5029(1)  X  9  5001(1).. 

8 

8 

13 

25 

8 
8 

X  9  5032(1).. 

X  9  5003(3).. 

Totals 

8 

27 

16 

12 

4 

3 

70 

1.80 

Table  16. — Grade  distribtdion  of  young  produced  by  Fz  does  {table  15)  or  back-cross  doe* 
{table  IJf)  mated  with  the  standard-bred  buck,  3036  {table  1/,). 


Parents. 

Grades  of  young. 

Totals. 

Mean 
lower 
group. 

Mean 
higher 
group. 

1  2 

.  1 
.  3 
1  . 

14 

3 

4 

1 

5 

4 

11 
5 

3 
2 

3 

24 

5 

2 
1 
3 

4 
1 
3 

14 

6 

3 
5 
4 
1 

2 
1 

16 

7 
2 

1 
3 
6 

2 
2 

9 

1 
2 

1 

4 

10 
1 

11 

12 

13 
1 

14 

1 
1 

15 

2 
2 
2 

10 

17 

cf  3036(9)  X  9  5150(5),  Fo.. 
X  9  5153(5),  Fj.. 
X  9  5170(6),  BC 
X  9  5166(4),  BC 
X  9  5158(5),  BC 
X  9  5159(6),  BC 
X  9  5169(6),  BC 

1 

8 
22 
18 

6 
12 

7 
14 

4.94 
5.27 

14.55 

Totals 

1 

1 

2 

6 

1 

87 

5.06 

14.55 

Table  17. — Grade  distribution  of  young  produced  by  second  back-cross  does  recorded  in 
table  16  and  the  same  standard-bred  buck,  3036  {tables  I4  and  16). 


Parents. 

Grades  of  young. 

Totals. 

Means, 
lower 
group. 

Means. 

upper 

group. 

1 

2 

3 

4 

5 

1 
1 

6 

2 
2 

7 

2 

1 

3 

S 

2 
5 

7 

9 

2 
2 

10 

11 

12 

13 

14 

15 

10 

17 

cf  3036(9)  X   9  5536(7),  2BC.. 
X   9  5590(9),  2BC.. 

3 

1 

1 

3 

2 

4 
21 

7.50 
8.00 

15.17 

Totals 

3 

1 

1 

3 

2 

25 

7.89 

15.17 

Table  18. — Grade  distribution  of  young  produced  by  F2,  back-cross  and  second  back-cross 
does  {tables  I4-I6)  mated  with  a  back-cross  buck,  c^ol67,  grade  7  {table  14),  «on  of 
&5003{S). 


Parents. 

Grades  of  young. 

Totals. 

Means, 
lower 
group. 

Means, 

upper 

group. 

2 

3 

1 

1 

2 

1 

5 

4 

1 
2 
2 

1 

1 

7 

5 

4 

1 
2 
3 
3 
2 
4 
2 

21 

6 

1 
1 

3 

2 
7 

7 
1 

1 
2 

8 

9 

10 

1 

1 

11 

12 

13 

14 
3 

3 

15 

2 
2 

4 

10 

17 

cf  5167(7)  X  9  5150(5),  Fj.  . .  . 
X  9  5153(3),  F2.  ..  . 

1 

6 
4 
9 
6 
6 
8 
6 
5 
4 

■   5.11 
4.00 

4.85 

14.40 
15.67 

X  9  5166(4),  BC... 
X  9  5158(5),  BC... 
X  9  5601(0),  2BC.. 
X  9  5645(6),  2BC.  . 
X  9  5920(6),  2BC.. 
X  9  5933(6),  2BC.. 
X  9  5936(5),  2BC.. 

1 

Totals 

2 

1 

53 

4.73 

14.87 

42 


HEREDITY   IN   RABBITS,    RATS,    AND    MICE. 

Table  19. — Variation  of  the  uncrossed  "white"  Dutch  race. 


Parents. 

Grades  of 
young. 

Totals. 

Means. 

15 

16 

17 

^?'(^^7'^(^7"l  y  Q5945('l5') 

1 
2 
3 
2 

1 

4 
15 

4 

1 

5 

7 
1 
1 
6 
3 
3 

1 
11 
25 

7 
1 
7 
3 
4 

}  16.17 

1 
^16.12 

16.71 
17.00 
16.75 

X  $7934(15) 

X  $6703(16) 

X  $7003(16) 

X  $7185(16) 

X  9  7313(17) 

cf  9218(17)  X  9  9222(15) 

X  $9217(16) 

Totals 

9 

24 

26 

59 

16.25 

Table  20. — Variation  of  the  uncrossed  "dark"  Dutch  race. 


Parents. 


'd^4 


X  9  7642(2). 

X  $8034(3). 

X  $7644(3). 

X  $7684(3). 

X  $6989(4). 

X  9  7685(4). 

X  9  8290(4). 

X  9  5153(5). 

X  9  6707(5). 


■  i    Totals. 


d' 6701  (5)  X  $7641(2). 


Grades 

of  young. 

Totals. 

Means. 

1 

2 

3 

4 

5 

6 

7 

o 

3 

3 

5 

2 

1 

7 
15 

8 
2 

4 

6 

4 

10 
10 
6 
8 
3 
3 

1 

2 
3 
5 

7 
9 
8 
3 

1 

3 
1 
2 

4 
3 

8 

1 

1 

18 

3 

12 

4 
26 
39 

27 

28 

'2 

5 

|3.05 

'3.35 

J 
1 

^3.35 
|3.20 

8 

44 

50 

39 

21 

9 

1 

172 

3.30 

Table  21. — Variation  of  the  ipicrossed  "tan"  Dutch  race. 


Parents. 

Grades  of 
young. 

Totals. 

Means. 

2 

3 

4 

5 

cf  5757(3)  X  $7393(3) 

cr7142(4)  X  9  9275(2) 

X  9  9608(3) 

X  $6424(3) 

X  $8881(3) 

X  $9044(3) 

c?6240(4)  X  $8881(3) 

2 
2 

1 

2 
8 
3 
2 
3 

3 

2 

1 
1 

4 
1 
3 

■   ■ 

1 
1 

4 

11 

6 

3 

7 
2 

7 

3.50 
2.91 

■3.40 

Totals 

5 

21 

12 

2 

40 

3.27 

TABLES. 


43 


Table  22. — Grade  distribution  of  the  Fi  young  produced  by  the  croaa  of  "white' 

inth  "dark"  Dutch. 


Parents. 

Grades  of  young. 

Totals. 

McauH. 

5 

6 

1 

1 
1 
3 

6 

7 

4 
3 
2 

9 

8 

2 
1 
1 
1 
2 

7 

9 

1 

1 

2 

10 

1 

1 

11 
1 

1 

cf  6175(17)  X   9  6666(5) 

X   9  5153(5) 

X   9  6705(5) 

X   9  9170(6) 

X   9  6038(8) 

1 

1 

o 

10 
6 
5 
5 
2 

[7.28 
[7.28 

Totals 

28 

7.28 

Table  23. — Grade  distribution  of  the  Fi  young  'produced  by  the  cross  of  "white"  uHth 

heterozygous  "dark"  Dutch. 


Parents. 

Grades  of  j'oung. 

Totals. 

Means, 
lower 
group. 

Means, 

higher 

group. 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

15 

16 

17 

c^6175(17)  X   9  5158(5).... 
X   9  5940(5)...  . 
X   9  5920(6)...  . 
X   9  5601(6).. .  , 

1 

1 

2 

1 
1 

2 
1 

1 

2 
2 

2 

1 

1 
1 

3 

1 

1 
2 
4 
2 
1 
1 

1 
1 
1 

1 

1 
2 

1 

1 
1 

1 
1 

1 

1 
1 

1 

•  • 
2 

1 

1 
1 
2 

3 
3 
3 

4 
1 
2 

1 
2 
6 
4 
2 

1 
1 

1 

1 

1 
3 
3 
1 

2 

10 

11 

6 

5 

5 

12 

14 

16 

15 

6 

5 

5 

3 

4 

8 

5 

^6.33 

6.82 

•7.57 
•  7.22 
•7.50 

7.25 

15.55 

15.67 

16.25 
15.50 
14.50 

14.72 

X   9  5939(6).... 
X    9  6570(6).... 
X    9  6643(7).. .  . 

2 

3 

2 

X   9  6891(7).. .  . 

X   9  6031(8)...  . 
X   9  7118(8)..  .  . 

1 

cf  7007(14)  X   9  5158(5) 

X   9  5940(5)...  . 
X   95601(6) 

1 

X   9  5920(6)...  . 
X   9  5645(6).. .  . 

1 

X    9  6570(6)..  .  . 

5 

7 

13 

Totals 

13 

15 

8 

2 

2 

1 

2 

8 

18 

24 

12 

130 

7.04 

15.56 

Table  24. — Grade  distribution  of  the  young  produced  by  a  cross  of  tan  Dutch  uith 

self  aniinals  heterozygous  for  tan. 


Parents. 

Grades  of  young. 

Totals. 

Means, 

Dutch 

young. 

0 

1 

2 

2 

4 

^^7142(4)  X   9  6000(0) 

X   96119(0) 

X   96122(0) 

X   9  6124(0) 

X   9  6380(0) 

X   97529(0) 

X   9  7677(0) 

X   98063(0) 

6 

16 

2 

2 

3 
5 

7 
3 

1 

1 
1 

1 
5 
2 

2 
1 

6 

8 
2 

1 
2 

1 
2 
2 

1 

3 

3 
2 

13 

31 

6 

6 

8 

10 

12 

5 

2.86 
2.60 
2.50 
3.75 
2.20 
3.20 
3.20 
3.00 

Totals              

44 

3 

11 

24 

9 

91 

2.83 

44 


HEREDITY    IN   RABBITS,    RATS,    AND    MICE. 


Table  25. — Grade  distribution  of  the  Fi  young  produced  by  the  cross  of 
dark  Dutch  with  tan  Dutch. 


Parents. 

Grades  of 
young. 

Totals. 

Means. 

0 

1 

2 

rti714'>('41T  X    97641(2)D   

2 

2 
4 

3 
1 
5 

2 
5 

16 

2 

2 

4 

5 
3 
5 
2 
9 

X    $6058(4)D 

c?6701(5)DX   $7209(2)T 

cf  5757(3)T  X   9  5170(6)D 

"             X    95939C6)D 

Totals 

24 

1.00 

Table  26. — Grade  distribution  of  the  F\  young  -produced  by  the  cross  of  white 

Dutch  with  tan  Dutch. 


Parents. 

Grades  of  young. 

Totals. 

Mean. 

6 

7 

8 

9 

10 

11 

12 

cf5757(3)T     X   97003(16)W 

X   97185(16)W 

d^6175(17)W  X    96424(3)T 

1 
1 

6 
3 

•• 

1 

•• 

•• 

3 

8 
4 
3 
5 

.... 

X    96539(4)T 

2 

2 

1 

•• 

•• 

•• 

Totals 

4 

11 

1 

1 

■• 

■• 

3 

20 

7.70 

Table  27. — Grade  distribution  of  the  Fi  young  from  crosses  of  dark  Dutch  with 

self  and  of  ivhite  Dutch  with  self. 


Parents. 

Grades  of 
young. 

Totals. 

Means. 

0 

1 

2 

3 

(Dark  X  self.) 
0^6701(5)  X    9 7413(0) 

3 
9 

1 

■• 

4 
9 

X   98012(0) 

Totals 

12 

1 

•• 

13 

.08 

(White  X  self.) 

c^6175(17)  X    9  61.33(0) 

X   9  7123(0) 

X   9  7124(0) 

X    98265(0) 

4 

11 

3 

5 

o 

4 

2 

2 

.... 

Totals 

IS 

13 

2 

33 

1.51 

TABLES. 


45 


Table  28. — Grade  distribution  of  the  Ft  young  from  the  several  crosses  made  between  the 
three  types  of  Dutch  and  between  white  Dutch  and  self. 


Cross. 

Grades  of  young. 

0 

1 

2 

3 

4 

5 

6 

7 

8  9 

10 

U 

12 

13 

14 

15 

4 
13 

16 

1 

2 

17 

Totals.  Means. 

White  X  self 

41 

45 
1 

22 

1 

14 
2 

1 
8 

9 
12 

7 
11 

5 

15 

12 
17 

5 
6 

13 

4 
5 

14 

ft   ft 

8 
4 

10 

4 

4 

12 

2 

8 
1 

10 

5 

1 

4 

6 
0 

4 

2 

191 

White  X  dark  (taVjles  16- 
White  X  tan 

-18). 

7   3 
lS'>ft 

56+25  5.828nd 
14.40 

130       

275 

Dark   X  tan 

89 

84 

25 

8   4  14   9 

1 

Table  29. — Grade  distribution  of  young  produced  by  other  crosses  of  Fi  animals. 


Cross. 

Grades  of  young. 

Totals. 

Means. 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

15 

16 
13 

7 
5 

17 
6 

2 
1 

Fi  (white  X  self)  X  white 

Fi  (white  X  self)X  self.. 
Fi  (darkX  self)  X  dark. 
Fi  (white  X  dark)  X  dark 
Fi  (white  X  tan)  X  white 
Fi  (dark  X  tan)  X  dark. 
Fi  (dark  X  tan)  X  tan.  . 
Fi  (dark  X  tan)  X  white 

65 
5 

4 

19 

1 

2 

26 

36 

3 

12 
9 
1 

3 

3 

9 

12 

6 
1 
5 

4 

11 

3 
10 
10 

11 

2 
36 

13 
23 

11 

33 

14 
14 

14 

30 

4 
5 

9 

12 

1 

4 

7 

3 

2 
2 

1 

1 

3 
5 

1 
3 

1 

8 

8 
4 

6 
12 

1 

13 

8 

14 

3 
10 

24 

18 
3 

58+58 

94 

59 
143 

73 

25 

76 
112 

5.15and 
15.15 

Table  30. — Grade  distribution  of  young  sired  by  the  original  English  male,  2545,  grade  2\. 


A.  Mother  English. 


Mother. 

Grade. 

Self 
young. 

English  young  of  grade — 

Total 
English. 

1 

u 

ih 

If 

2 

2\ 

2^ 

•21 

3 

7 

2649 
2650 
2651 

2 
3 
2f 

1 
5 
2 

2 
2 

1 

1 
2 
1 

2 

1 

1 

1 
1 

4 
2 

3 

13 
5 

Totals .  .  . 
Means  .  . 

8 

4 

1 

4 

2 

1 

1 

2 

6 

21 

1.0. =5 

2.32 

B. 

Mother  S 

ELF. 

Mother. 

Self 

English  young  of  grade — 

!    Total 

young. 

Ih 

li 

2 

2J 

o3 
-4 

3 

3i 

3J 

3f 

4 

/ 

English. 

1443 

2 

.. 

3 

3 

1492 

5 

1 

1 

3 

5 

2053 

2 

.  . 

5 

5 

2502 

5 

1 

2 

3 

2867 

3 

3 

3 

2912 

0 

2 

1 

1 

1 

.  . 

5 

2916 

1 

•• 

•• 

1 

1 

O 

Totals .  .  . 
Mean  . . 

18 

2 

■  • 

1 

■■ 

1 

•• 

1 

2 

1 

1 

17 

26 

2.80                                       1 

46 


HEREDITY   IN   RABBITS,    RATS,   AND   MICE. 


Table  31. — Grade  distribution  of  young  sired  by  English  male  2711,  grade  4,  generation  1, 

ancestor  of  all  English  in  the  selection  series. 
A.  Mother  English. 


Mother. 

Grade. 

Gener- 
ation. 

Self 
young. 

English  young  of  grade — 

Total 
English. 

Means, 
higher 
group. 

3  1 

4  J 

1  . 

H 

U 

lf22i 

2i 

2|c 

i3i 

3^ 
1 

1 

2 

3|4 

1 

1 

2 

I4i 

4^ 

4f5 



6051 
5052 
6206 
6083 
5084 
6402 

3h 

3h 

4f 

5 

5 

5 

2 
2 
0 
1 
2 
1 

1  ] 
1   . 

1 

1 

1 

1 
1 
1 

3 

.  .  1 
.  .  1 

4 
4 
6 
4 
3 
3          5 

js.ss 

3.80 
4.25 

.  .  ] 
.  .  ] 

[.  . 
i.  . 

2 

1 
2 
1 

Totals  . 

8 

1] 

L  2 

4 

2  1.. 

1 

3        26 

Mpanfl 

1.52 

3.93 

B.  Mother  Self. 


Mother. 

Self 
young. 

English  young 

of  grade — 

Total 
English. 

3 

3i 

3^ 

3f 

4 

4i 

4^ 

41 

5 

2765 

1 

2 

1 

3 

2770 

1 

2 

.. 

1 

2 

5 

2840 

4 

1 

1 

1 

^ 

3 

2862 

4 

,  , 

1 

1 

2 

2867 

3 

1 

1 

,   , 

1 

, 

2 

5 

2878 

0 

2 

1 

_  , 

.  , 

3 

2947 

4 

,  , 

.  , 

2948 

4 

,   , 

,   , 

,   , 

2 

1 

.   , 

3 

2983 

4 

1 

1 

,   , 

,  , 

. . 

1 

3 

3019 

3 

1 

•• 

•• 

•• 

•• 

1 

Totals .... 
Mean 

28 

3 

5 

4 

2 

5 

3 

1 

5 

28 

3.89 

Table  32. — Grade  distribution  of  young  sired  by  English  male  6086,  grade  4h  generation  1  \. 

A.  Mother  English. 


Mother. 

Grade. 

Gener- 
ation. 

Self 
young. 

English  young  of  grade — 

Total. 
English. 

1 

2 

3 
1 

1 

li 

l\ 

If 

2 

2i 

2\ 

2f 

3 

3i 

3i 

3f 

4 

4t 

4i 

4f 

5 

5051 

3| 

U 

2 

1 

1 

1 

1 

1 

1 

1 

1 

1 

9 

5052 

3i 

U 

8 

2 

1 

1 

2 

1 

2 

2 

2 

2 

2 

18 

5188 

4i 

2h 

2 

,    . 

^ 

1 

1 

5206 

4f 

2h 

2 

1 

1 

2 

4 

5053 

5 

n 

2 

1 

2 

1 

4 

5083 

5 

n 

6 

_ 

1 

1 

2 

1 

.    , 

6 

5084 

5 

u 

3 

1 

1 

3 

5101 

5 

u 

3 

1 

2 

1 

1 

1 

1 

1 

3 

1 

12 

5102 

5 

u 

6 

1 

1 

1 

1 

1 

5 

5398 

5 

If 

1 

1 

8 

1 

2 

Totals  . 
Means . 

35 

1 

3 

5 

4 

4 

3 

3 

4 

5 

8 

4 

5 

6 

1 

64 

1  ss 

.■^  Qfi 

TABLES. 


47 


Table  32,  continued, 
B.  Mother  Self. 


Mother. 

Self 
young. 

English  young  of  grade — 

ToUl 
Engliih. 

li 

li 

li 

2 

2i 

2i 

2J 

3 

3J 

3i 

2 

1 
3 

3} 

4 

1 

1 

4i 

4i 

4J 

5 

4146 
4147 
4148 

3 
2 
3 

1 

1 

1 

1 

1 

•• 

1 

7 
1 
2 

Totals.  . .  . 
Mean .... 

8 

1 

1 

•• 

1 

2 

1 

10 

.•?  Mi 

Table  33. — Grade  distribution  of  young  sired  by  English  male  6S76,  grade  4\,  generation  £\. 


Mother. 

Grade. 

Gen- 
era- 
tion. 

Self 
young. 

English  young  of  grade — 

Total 
Eng- 
1    lish. 

Means. 

1 

1 
2 

1 
1" 

Lli 
I  1 

■  1 

1 

1 

2 

lf2 

2 
. 

. .  1 

2  2 

2i 

1 

2f 

1 

1 

3 

1 
1 

2 

3J 

1 

1 

3i 

1 

1 

1 
3 

33^ 

1 

1   ] 
1    . 

1  1 

1  ': 

2  . 
2   . 

9  i 

14i 

4J 

1 

4f£ 

5051 
5052 
5188 
5205 
5206 
5083 
5084 
5102 
5398 

3i 

3^ 

4i 

4i 

4i 

5 

5 

5 

5 

2^ 
2^ 
2J 

11 
If 

2 
2 

1 
1 
1 
2 
5 
9 
2 

6 

8 
4 
5 
4 
1 
6 
8 
4 

I3.4O 

js.ss 

3.75 
4.11 

1 

J  .  . 

5  .  . 

1 

..  1 
..  1 

I  .  . 

2 

.  .  2 

Totals   . 

25 

46 

Means  . 

1.20 

3.7Q 



Table  34. — Grade  dist 

•ibution  of 

young 

sired  by  English  male  5555,  grade 

41,  generation  2\ 

Mother. 

Grade. 

Gen- 
era- 
tion. 

Self 
young. 

English  j'oung  of  grade — 

Total  Means, 

-     IT.     .          1-  -•     t    _ 

1 

2  " 

1  ] 

\  1 
1 
1 

2 
2 

2 

2 

I  .  . 

2 

U 

1 
1 

1 

2 
3 
1 

1 
2 

1 
1 

1 
1 

■ 

If 

1 

2 
1 

1 
1 

2 

1 

1 

2i 

2h 
1 

2f; 

j|3i 

1 
I.. 

1 
1 2 

3i 

2 

1 

2' 

1 
1 

7 

3f 

1 

1 

1 
1 
1 

1 

1 

'_' 
7 

4 

1 
5 

2 

1 

4 

1 

1 
1 

1 

4i 

3 
2 

2 
1 

2 

4J 

2 

1 

■3 

1 

3 

4f 

1 
1 

1 
2 
1 
1 

1 

5 

i 

1 
1 
5 

(Eng- 
.'  lish. 

Qigner 
group. 

5557 
5051 
5052 
5561 
5701 
5752 
5993 
5188 
5205 
5672 
5769 
5952 
6074 
5206 
5793 
5801 
5988 
5084 
5102 
5398 
5951 

3 

31 
3^ 
3i 

4 

4 

4 

4i 

4i 

4^ 

4^ 

4^ 

4i 

4f 

4f 

4S 

4^ 

5 

5 

5 

5 

2i 

ih 

2i 

3i 

3 

3 

2i 

2^ 

3 

3i 

3 

3i 

2^ 

3i 

3 

3 

If 

3 

3 
3 

2 

1 

1 
9 
1 
6 
2 
1 
2 

6 
2 
3 
1 
12 
2 
7 
1 

7 

I      11 

3 

1 

11 
5 
2 

19 
3 
[       6 
3 
5 
5 

22 
8 

15 
4 

15 

12 
9 
3 

4.04 
4,47 

4.44 

1  ] 

i 
1 

3 
3 

2 

4 
2 

2 
3 
6 

4 

2 
2 

2 

2  . 

1  . 

2  . 
2  . 
1  . 

Totals 

65 

I2J9 

7 

6 

4 

1 

. .  ] 

isjio 

2' 

168 

Aleans 

1 

"* 

1.36 

4.40 

' 



48 


HEREDITY   IN   RABBITS,    RATS,    AND    MICE. 


Table  35. — Grade  distribution  of  young  sired  by  English  male  6S70,  grade  5,  generation  S. 


Mother. 

Grade. 

Gen- 
era- 
tion. 

Self 
young. 

English  young  of  grade — 

Total 

Eng- 
lish. 

Means, 

higher 

group. 

1 

li 

1§ 

If 

2 

2i 

2| 

2| 

3 

3i 

3^ 

3f 

4 

4i 

4i 
^2 

4f 

5 

5752 
6993 
5188 
5672 
5769 
6074 
6089 
7193 
6815 
6369 
6417 
6693 
6788 

4 

4 

4i 

4^ 

4J 

4^ 

4^ 

4^ 

4| 

5 

5 

5 

5 

3 
3 

2\ 

3 

3i 

3i 

3^ 

4i 

3i 

3 

3 

4i 

4i 

4 
2 
0 
0 
0 
4 
2 
0 
3 
1 
1 
1 
3 

l' 

1 

2 

1 

1 
1 

1 

1 

3 
1 

1 

1 
1 

1 
1 

1 

•  ■ 

1 

1 
3 
1 

2 

1 

2 
2 

1 
2 
4 
2 

1 
1 

1 
2 

1 

1 
1 
2 
1 

2 

4 
1 
1 
1 

2 

2 
5 

1 

1 

4 
7 
7 
1 
7 
2 
7 
4 
17 
4 
3 
7 
2 

■4.67 
4.63 

Totals  . 

21 

1 

3 

3 

2 

5 

2 

2 

1 

1 

8 

15 

11 

18 

72 

Mean 

1.79 

4.0(1 

Table  36. — Grade  distribution  of  young  sired  by  English  male  6420,  grade  4  h  generation  3. 


Mother. 

Grade. 

Gener- 
ation. 

Self 
young. 

English  young  of  grade — 

Total 
English. 

1 

n 

I5 

If 

2 

9i 

-4 

2^ 

2| 

3 

H 

3h 

o3 

4 

4i 

^ 

4f 

5 

5993 

4 

3 

1 

1 

•    • 

1 

2 

4 

5188 

4i 

2i 

0 

2 

2 

1 

,    , 

5 

6074 

4^ 

3i 

0 

1 

3 

2 

1 

7 

5102 

5 

H 

1 

1 

1 

1 

1 

1 

5 

6417 

5 

3 

1 

_ 

1 

1 

1 

3 

6841 

5 

41 

1 

1 

1 

2 

1 

5 

Totals  . 

4 

2 

2 

2 

1 

1 

1 

1 

2 

1 

5 

1 

3 

5 

2 

29 

Means 

i.sn 

4..'^.'^ 

Table  37. — Grade  distribution 

of 

your 

9 

sired  fey  English 

mi 

iZe  6071 

grad 

e5 

,  generation  S  | 

Mother. 

Grade. 

Gen- 

Self 

English  young  of  grade — 

Total 
Eng- 

Means, 
higher 

era- 

1    1     1 

tion. 

young. 

1 

n 

u 

li 

2 

9191 

2f 

3 

3i 

H 

3f 

4 

4i 

4| 

4| 

5 

lish. 

group. 

5051 

3^ 

H 

1 

2 

3 

5 

3.75 

5752 

4 

3 

1 

1 

1 

1 

1 

2 

6 

4.56 

5769 

4i 

3^ 

0 

1 

1 

2 

4 

4.83 

5891 

^ 

3* 

2 

1 

1 

1 

3 

•4.25 

6074 

^ 

3^ 

1 

1 

1 

1 

1 

1 

5 

5206 

4f 

2^ 

0 

1 

2 

1 

4 

5793 

4| 

3i 

2 

1 

4 

5 

5988 

4f 

3 

3 

2 

1 

1 

1 

2 

7 

[4.47 

6073 

4f 

3^ 

1 

1 

_ 

2 

1 

4 

6189 

4f 

3 

1 

2 

1 

1 

4 

5084 

5 

H 

2 

1 

1 

1 

1 

4 

\  .  ,0 

5951 

5 

3 

2 

1 

2 

3 

U.12 

Totals  . 

16 

4 

3 

3 

7 

3 

2 

3 

5 

2 

5 

3 

6 

8 

54 

Means. 

1  fii 

A  50 

TABLES. 


49 


Table  38. — Grade  distribution  of  young  sired  by  English  male  6072,  grade  5,  generation  S\. 

A.  Mother  HETEnozraous  English. 


Mother. 

Grade. 

Gen- 
era- 
tion. 

Self 
young.   . 

English  young  of  grade — 

Total 
Eng- 
lish. 

Means, 
higher 
group. 

ill 

I' 
L  . 

lili 

IJ 

2 

2i 

2i 

2iC 

1  ! 

2  ! 

1 

1 

(3i 

3J 

3jU!4i 

4i!4j 

5 

2 

1 
1 

,    . 

2 
2 

1 

1 
3 
4 

1 

2 

1 
2 
2 
1 

2 
5 

1 
1 
1 

61 

5752 
5701 
5993 
5188 
5769 
5891 
6074 
6079 
6089 
6452 
6795 
5206 
5801 
5988 
6189 
6264 
7450 
7817 
8813 
5084 
5102 
6188 
6369 
6416 
6417 
6622 
6693 
6841 
7476 

4 

4 

4 

4i 

4i 

4^ 

4^ 

4-^ 

4^ 

4| 

4^^ 

4f 

4f 

4f 

4| 

4f 

4f 

4f 

41 

5 

5 

5 

5 

5 

5 

5 

5 

5 

5 

3 

3J 

3 

2^ 

3i 

3^ 

3| 

4 

3J- 

4 

3| 

2h 

3 

3 

3 

3f 

4^ 

3^ 

4f 

ih 
ih 

3 

3 

3 

3 

3i 

4i 

4i 

4i 

o 

3 
2 
0 

4 
3 
2 

1 
7 
2 

3 
3 

4 
6 
4 
6 
5 
1 
0 
2 
9 
1 
2 
2 
4 
1 
0 
2 
0 

1 

1 

1 
2 

V 

2 

1 

1 
1 

1 

1 

1 
1 

1 

i 
1 

■ . 

1 
1 
1 

2 

2 

2 
1 

1 
2 

1 

3 

1 

2 

2 

1 

1 

1 

1 

1 

1 
2 

•    • 

2 

1 

4 

1 
1 

1 

1    1 
1    1 

1 

1 
1 

1 

5 
1 

2 
3 

1 
1 

1 
4 
3 

4 

1 

1 

4 

1 
1 

3 

1 
1 
4 
4 

2 
2 
3 

1 

1 
1 
4 
1 
1 
4 
3 
3 
4 
7 
4 

J 

1  4.50 

[4.42 

4.58 
4.62 

4.69 

2 

1 

i'.. 
I 

1 

I 

12 

4 

8 
12 
12 

1 
11 
15 
13 

5 

1 

3 
4 

7 
16 

7 
10 
22 

5 

5 
12 
19 

4 

2 

1 

3 
3 

1 
1 

1 

1 
1 
2 

1 

1 
1 

2 

1 
2 

Totals.. 

81 

12 

8 

7 

16 

15 

9 

8 

5  ' 

1  .  . 

3 

13 15 

35 

56 

36 

1 

234 

Means  . 

1 

1 

1.97 

4.63 

B.  Mother 

Homozygous 

Enolise 

[. 

Mother. 

Grade. 

Gener- 
ation. 

Self 
young. 

English  young  of  grade — 

Total 
English. 

1 

U 

U 

1^ 

2 

0.1 
-4 

2| 

o3 

-4 

3 

31 
1 

3J 

1 
1 

3! 

1 
1 
2 

4 

2 

1 

3 

4i 

4J 

4J 

2 

2 
4 

5 



1 
1 

5733 
7535 
7814 
8704 

2 
U 

ll 

3 
4 
3^ 

4i 

0 
0 
0 
0 

1 

2 

1 

1 

2 
2 

1 

*    ' 

3 

1 

1 
1 

8 

1 

14 

5 

Totals .  .  . 

1 

2 

1 

1 

4 

1 

4 

2 

1 

28 

Means.  .  . 

2.11 

4.17 

C. 

Mother  Self. 

Mother. 

Self 
young. 

English  young  of — 

Total 
English. 

3  J 

n 

4 

4i 

4J 

4J 

7123 

7878 

6 
3 

1 

. , 

1 

1 

2 

1 
1 

3 
4 

Totals .... 
Mean .... 

9 

1 

1 

1 

2 

2 

7 

4.32 

50 


HEREDITY   IN   RABBITS,    RATS,    AND   MICE. 


Table  39. — Grade  distribution  ofyov 

ing 

Sired  fcy  English 

7nale  696. 

'f, 

grade  5,  generation  4h 

Mother. 

Grade. 

Gen- 
era- 
tion. 

Self 
young. 

English  young  of  grade 

Total 
Eng- 
lish. 

Means, 

higher 

group. 

3 
4 

1 

li 

H 

If 

2 

■21 

2^i2f 

3 

3i 

3^ 

3f 

4 

4i 

^ 

4f 

5 

6074 
7193 
7475 
7356 
7817 
5102 
6417 
6693 
6841 

4f 

4| 

4f 

5 

5 

5 

5 

4i 
4i 
4^ 
3i 
li 
3 

4i 
4i 

4 
2 

1 
2 
1 

1 

2 

4 

1 
1 

1 

1 

1 
1 

1 

2 
1 

1 

2 

1 

1 

1 

1 
1 

2 

1 

1 

1 

*  ' 

*    * 

1 

4 
1 

2 

1 

2 
1 

3 
2 

2 

1 

2 
2 
•  • 

1 

2 

8 
4 
3 
2 
8 
11 
6 
6 
2 

I4.64 

■4.78 

< 

•4.65 

Totals. 

21 

2 

4 

4 

4 

1 

3 

2 

1 

■  • 

•  • 

2 

1 

8 

10 

8 

50 

Means. . 

l".'49 

4.68 

Table  40. — Grade  distribution  of  young  sired  by  English  male  7699,  grade  5,  generation  4h 

A.  Mother  Heterozygous  English. 


Mother. 

Grade. 

Gen- 
era- 
tion. 

Self 
young. 

English  young  of  grade — 

Total 
Eng- 
,   lish. 

Means, 

higher 

group. 

Lli 

l§ 

If 

2 

2i 

2§ 

2f 

3 

3i 

3^ 

3f. 

t4i 

4i 

4f 

5 

5i 

9449 
6089 
5988 
6795 
7300 
7356 
7389 
7475 
7817 
9350 
5084 
6369 
6416 
6417 
6622 
6841 
7325 
7476 
7903 
8257 
8259 
9349 
9363 
9091 
9535 

31 

4i 

41 

4f 

4f 

4f 

4f 

4f 

41 

4f 

5 

5 

5 

5 

5 

5 

5 

5 

5 

5 

5 

5 

5 

5 

5i 

3^ 
3i 
3 

3^ 
4i 
4i 
4i 
4i 
3^ 
5i 

3 

3 

3 

3f 

3f 

3^ 

4i 

4 

4| 

4f 

o\ 

5 

5i 

3i 

2 

2 

1 
4 
0 
0 
0 
8 
1 
2 

2 
4 
3 
4 
5 
3 
0 
3 
3 
0 
7 
0 
0 
3 
7 

1 

1 

2 

1 

1 
1 

1 

1 

1 
1 
1 

2 

1 

14 

3 
1 
1 

1 
3 

1 
10 

1 
2 

1 

1 

6 

1 

1 

2 

1 

1 

2 
2 

1 
11 

2 

1 

1 

3 

3 

2 
13 

1 
1 

2 

1 
1 

2 

1  . 
1  ] 

1 

3  i 

2 

1 

L  1 

1 
1 

I  6 

1 

1 

1 

1 
4 

1 

1 

1 

1 
1 

3 
1 

1^ 

3 

4 

5        8 

6 

10 

1  3 
3 

17 

6 

17 

9 

10 

11 

L      10 

9 

20 

3 

8 

9 

.       16 

.       15 

2  16 
I         5 

2 

4 

4.50 
4.62 

■4.66 

•4.80 
>4.S1 

1 

1 
2 
1 

3 

1 
2 
5 
5 
3 
2 
3 

1 
3 

2 
4 
1 
3 

43 

1 

1 

1 
1 
1 
1 
1 
1 
3 
6 
2 
2 
1 
1 
3 
4 
1 
5 
2 
5 
6 
6 

1 
2 

re 

1 

.  .  ] 

1 

2 
1 

1 

2 

8 

.   1 

2 

1 

1 

I  .  . 

2 

1 

2 

1 

1 

1 

L  5 

5 

6 

Totals  . . 

64        ] 

3    224 

Means . . 

•^  ?.^                              4  ><n 

TABLES. 


51 


Table  40,  continued. 
B.  Mother  Homozygous  Enolirh  or  Self. 


Mother. 

6 

2 
O 

Self 
young. 

English  young  of  grade — 

-r. 

3 

IJ 

H 

If 

2 

1 

1 

2J 

2J 

3 

1 

3i 

3J 

3J 

4 
1 

4i 

1 
2 

4J 
2 

4J 

3 

8 

5 

1 
0 

5i 

1 
1 

C     u 

11 

16 

3 

*5^  6e 

Horn.  Eng.,  8704 

Self,  7124 

"     8251 

H 

4f 

3 

2 

1 

4.78 

Table  41 

— Grade  distribution  o 

/  young  sired  btj  English  male  95S2,  grade  5, 

generation  4. 

Mother. 

d 

o 

1 
English  young  of  grade —                  "?.  .^ 

1  - 

cn    u    „• '  «    u    _• 

g  is  i   ?-S  ? 

>>li 

a  If  2 

21 
1 

2^ 

2f  3 

31  3i  3|  ^ 

I4i4i 
..    1 

4J5 
2  . 

5i^W 

«  0  £ 

s:s& 

Het.  Eng 

6795 

4f 

3^ 

3    1 

5 

7475 
9362 

4| 
4f 

4i 
5 

1  .  . 

2  .  . 

2 

1    '. 

.  .    1    .  . 

..    1 

3  . 
1  . 

4 
5 

2.46 

4.69 

9538 

4| 

4| 

1    .  . 

•    .     .    . 

1  . 

1 

6369 

5 

3 

1    .. 

I  .  .  .  . 

.  .  2 

..      4 

6622 
8257 

5 
5 

3f 

4| 

0    .. 
2    .  . 

..    1    . 

2    . 

.  .   2 

2  . 
1  1 

..      5 
4 

2.30 

4.70 

8259 

5 

4f 

0    .  . 

1 

.  .    1 

4  . 

6 

9535 
9592 

5i 
5^ 

3i 

4i 

0    .  . 
0    .  . 

.  .   ..  ] 

)   2 

2 

1   1 

4    1 

2   ..  .. 
2    1    .. 

L  ..   5 

1  . 
1  1 

16  4 

1       4 
..      5 

2.85 

4.94 

Totals. 

Means. 

Horn.  Eng. 
Self, 

10    1 

. .  1  :: 

1     43 

2.53 
1.75 

4.73 
4.30 

2.53                    1 

4.7: 
I  1    1 

1 

6 

7814 

li 

?* 

. .  1 

L 

8251 
8265 

3    .  . 

.  .    1 

2   . 

?. 

..      3 
2 

} 

4.80 

3    .  . 

Table  42. — Grade  distrihudon  of  7/oung  sired  by  English  male  9S06,  grade  5,  generation  5  J. 


Mother. 

Grade. 

Gener- 
ation. 

Self 
young. 

English  young  of  grade — 

Total 
English. 

2 
1 

2\ 

3 

1 

2h 
1 

2| 

1 

3 

2 
1 

3i 

3J 

3| 

4 

4i 

1 

1 

4J 
1 

1 

4J 

1 
1 

1 

1 

4 

5 

2 
4 

1 
1 
2 

10 

6844 
9871 
6369 
6622 
456 

3i 

4f 

5 

5 

5 

4i 
4 
3 
3f 

4i 

2 
2 
1 
1 
1 

5 
9 
3 
5 

4 

Totals.  .  . 

7 

1 

4 

1 

1 

3 

26 

Means. .  .  . 

2.52 

4.S6 

52  HEREDITY   IN   RABBITS,   RATS,   AND   MICE. 

Table  43. — Grade  distribution  of  young  sired  by  English  male  1212,  grade  5,  generation  5. 

A.  MoTHEB  Heterozygous  English. 


Mother. 

Grade. 

Gener- 
ation. 

Self 
young. 

English  young  of  grade — 

Total 
English. 

Means 
higher 
group. 

2 

2i 

2^ 

2f 

3 

3i 

•J  2 

3f 

4 

4i 

4i 

4f 

5 

5i 

9943 

455 

539 

1360 

6795 

7475 

8257 

452 

9372 

230 

6841 

537 

6622 

112 

1222 

596 

1613 

1860 

6369 

9363 

111 

597 

4| 

4f 

4f 

4f 

4f 

4f 

5 

5 

5 

5 

5 

5 

5 

5 

5 

5 

5 

5 

5 

5 

5i 

5i 

5i 

4i 

5i 

51 

3i 

4| 

4f 

5f 

4 

4| 

4i 

5J 

31 

5f 

5i 

51 

4i 

6^ 

3 

5 

5f 

5| 

2 

1 

7 
4 

1 
2 

1 

1 
2 

4 

2 

1 
1 

1 

1 

1 

2 

1 
1 

1 

1 

1 

1 

2 

1 

2 
1 

1 
1 

1 

1 

1 

1 
1 

1 

1 

1 

2 

1 

1 
2 

1 

1 

1 
1 

4 
2 

5 
2 
2 
2 

4 
3 
3 

1 
1 

2 
1 
1 
5 
2 

1 
2 

2 
1 

* 

1 
3 

5 
3 
4 

7 
5 
4 
5 
1 
5 
5 
4 
3 
2 
7 
1 
3 
7 
3 

3 

8 
2 

■4.97 

■4.97 
>5.15 

Totals  .  .  . 

29 

7 

1 

5 

4 

2 

2 

3 

2 

11 

43 

7 

87 

Means  .  .  . 

2.87 

4.Q8 

B.  Mother  Homozygous  English. 


Mother. 

Grade. 

Gener- 
ation. 

English  young  of  grade 

Total 
English. 

If 

2 

2i 

oi 

-2 

-4 

3 

3i 

31 

3f 

4 

4i 

4^ 

4f 

5 

8682 
9953 
9684 
9970 

If 
2f 
3| 

4f 
5 

4§ 
4| 

1 

1 

1 

1 

4 

1 
1 

1 

1 
1 

2 

3 

3 

2 

11 
3 
3 
6 

Totals  .  .  . 

1 

1 

1 

1 

4 

2 

1 

7 

5 

23 

Means. . . . 

'^..'Sfl 

4  7E 

1 

TABLES. 


53 


Table  44.— 


Grade  distribution  of  young  sired  by  English  male  6S4,  grade  SI  generations 
A.  Mother  HEXERozYaois  Knolish. 


I 


Mother. 

Grade. 

Gener- 
ation. 

Self 
young. 

English  young  of  grade — 

Total 
Englioh. 

Mean*, 

higher 

group. 

0 

21 

2J 

3 

1 
3 

1 
2 

1 

2 

3i 
* 

1 

1 

1 

3i 
1 

2 
1 

31 

1 
1 

1 

4 

1 

4i 

1 

4i 

4| 

1 

1 

3 

1 

1 

1 
1 

6 

2 

1 
1 
2 

3 
2 
1 
5 

2 
1 

1 
2 

2 
2 
3 
3 
3 
2 

1 
3 

5i 

2 

1 
2 

1 
2 

1 

9684 

9943 

7475 

9871 

360 

455 

584 

1360 

539 

6369 

9363 

7906 

258 

452 

230 

456 

537 

1222 

6841 

112 

9091 

2325 

111 

597 

9592 

3J 
4-J 

4J 
4| 
4| 
4i 

4i 

41 

0 
5 
5 

5 

5 

5 

5 

5 

5 

5 

5 

5 

5i 

5i 

5i 

5i 

5i 

4\ 

4 

5i 

4\ 

5k 

5i 

5i 

3 

5 

4 

5J 

4i 
5\ 
5} 
4i 
5f 
5i 
5i 
5f 
5f 
4i 

2 

1 
0 

2 

'o 
2 

1 

1 

2 
1 

1 

1 

1 
1 

1 

1 

1 

1 

2 

'  ■ 

•  • 

1 
2 

1 

1 

1 

1 
3 
4 

2 

4 
6 
2 
5 
5 
5 
7 
2 
6 
3 
2 
4 
1 
2 
2 
5 
5 
6 
4 
3 
5 

4.87 

5.02 
5.06 

Totals  .  .  . 

21 

5 

6 

11 

3 

4 

3 

1 

1 

9 

42 

9 

94 

Means .  .  . 

'>'un 

4  07 



B.  Mother  Homozygous  English. 


Mother. 

Grade. 

Gener- 
ation. 

English  j'oung  of  grade — 

Total 
English. 

u 

H 

li 

2 

21 

3 

3\ 

3i 

■31 

4 

4i 

4J 

4J 

5 

7814 
8682 
9953 

If 

2f 

3^ 
4f 
5 

1 

1 

1 

1 

2 

' 

1 
3 

1 
1 

1 
1 

6 
5 
3 

Totals .  .  . 

1 

1 

1 

1 

2 

4 

2 

2 

14 

Means .  .  . 

2.37 

4.69 

54 


HEREDITY   IN   RABBITS,    RATS,    AND   MICE. 


Table  45. — Giade  distribution  of  young  sired  by  homozygous  English  male  1173,  grade  S\, 

generation  5. 

A.  Mother  Heterozygous  English. 


Mother. 

Grade. 

Gener- 
ation. 

English  young  of  grade — 

Totals. 

n 

If 

2 

2i 

2J 

2i 

3 

3i 

3i 

3f 

4 

4i 

4^ 

4f 

5 

5i 

9684 
9970 
7300 
455 
584 
6369 
8257 
1222 

3i 

31 

4| 

4f 

4f 

5 

5 

5 

4§ 

^ 

4i 

4 

3 

4f 
5i 

1 

1 
3 

2 

1 

2 

1 
2 
1 

2 

1 

1 

1 

1 

1 

2 

2 

2 

1 
1 

3 
1 

3 

1 
3 
1 

3 

1 

5 
8 
8 
5 
8 
5 
3 
4 

Totals  .  .  . 

1 

4 

5 

4 

2 

4 

2 

2 

2 

2 

2 

7 

8 

1 

46 

Means  .  .  . 

2.42 

4  77 

B.  Mother  Homozygous 

English. 

Mother. 

Grade. 

Gener- 
ation. 

English  young 
of  grade — 

Totals. 

2 

2\ 

21 

2f 

3 

7814 
9953 

2f 

3^ 
5 

1 

9 

1 

2 

4 

4 
6 

Totals  .  .  . 

1 

2 

1 

2 

4 

10 

Mean .... 

2  fi.i^ 

C. 

Mother  Self. 

Mother. 

English  young. 

Totals. 

4i 

^ 

4f 

5 

5J 

7123 
7124 
8251 
9318 

1 

2 

2 

4 
3 
2 

2 
9 

5 

1 

8 

16 

2 

5 

Totals .  .  . 
Mean. . .  . 

1 

4 

9 

16 

1 

31 

4.84 

TABLES. 


00 


Table  46. — Summary  of  young  of  selected  heterozygous  English  males  by  heteroeygous 

English  females. 


Homo- 

Hetero- 

Un- 

Sire. 

Gener- 

Self 

zygous 

Mean 

zygous 

Mean 

graded 

T   •   • 

-::  •  ,1 

ation. 

young. 

English 
young. 

grade. 

English 
young. 

grade. 

K..gli.»h 
young. 

1. 

•C- 

2711 

4 

1 

8 

12 

1.52 

11 

3.93 

3 

26 

34 

50S6 

H 

H 

35 

20 

1.38 

44 

3.96 

64 

99 

5375 

^ 

2^ 

25 

17 

1.20 

29 

3.79 

46 

71 

5555 

4f 

2i 

65 

41 

1.36 

125 

4.40 

o 

16)> 

233 

6370 

5 

3 

21 

18 

1.79 

54 

4.60 

72 

93 

6420 

4| 

3 

4 

10 

1.80 

19 

4.33 

29 

33 

6071 

5 

3^ 

16 

22 

1.01 

32 

4.39 

54 

70 

6072 

5 

3^ 

81 

75 

1.97 

159 

4.63 

1 

235 

316 

6964 

5 

44- 

21 

21 

1.49 

29 

4.08 

50 

71 

7699 

5 

4i 

64 

75 

2.31 

149 

4.80 

8 

232 

296 

9532 

5 

4k 

10 

16 

2.53 

27 

4.73 

43 

53 

9S06 

5 

5^ 

7 

10 

2.52 

16 

4.86 

26 

33 

1212 

5 

5 

29 

26 

2.87 

61 

4.98 

87 

116 

534 

5i 

5 

21 

32 

2.95 

62 

4.97 

94 

115 

Totals .  . 

407 

395 

817 

14 

1.226 

1,633 

Percent . 

24.9 

24.2 

50.0 

0.9 

75.1 

Table  47. — Line  of  advance  in  the  selection  experiment  unth  heterozygous  English  males. 


Sire. 

No.  of 
homo- 
zygous 
young. 

Mean  of 
homo- 
zj-gous 
young. 

No.  of 
hetero- 
zygous 
young. 

Mean  of 
hetero- 
zygous 
young. 

Advance, 
homo- 
zygous 
young. 

Advance, 
hetero- 
zygous 
young. 

2545 

5 
12 
41 
81 
75 
58 

1.05 
1.52 
1.36 
1.97 
2.31 
2.91 

9 

159 
149 
123 

2.80 
3.93 
4.40 
4.63 
4.80 
4.97 

^47 
-.16 
.61 
.34 
.60 

Li3 
.47 
.23 
.17 

.17 

2711 

5555 

6072 

7699 

1212  and  534 

Total  advance .  .                  .... 

Average  advance ...            .  .                  .... 

.... 

1.86 
.37 

2.17 
.43 

Table  48. — Relative  frequency  of  cross-overs  among  the  gametes  formed  by  Fi  rats  of  the 

two  sexes. 


Series. 

Fi  parent. 

Dark-eyed 
young. 

Red-eyed  or 

pink-eyed 

yellow 

young. 

Total 
young. 

Percentage 

dark-eyed 

young. 

Percent 

Repulsion 

Coupling 

Female.  . .  . 

Male 

Female.  . .  . 
Male 

101 

73 

699 

556 

837 

703 

1,046 

731 

938 

776 

1,745 

1.287 

lO.S 

9.4 

40.0 

43.2 

21.5 
18.8 
19.8 
13.5 

56  HEREDITY   IN   RABBITS,    RATS,    AND    MICE. 

BIBLIOGRAPHY. 

Castle,  W.  E.,  and  P.  B.  Hadley. 

1915.  The  English  rabbit  and  the  question  of  MendeUan  unit-character  constancy. 
Proc.  Nat.  Ac.  Sci.  1,  pp.  39-42,  6  figs. 

and  John  C.  Phillips. 

1914.  Piebald  rats  and  selection.    Carnegie  Inst.  Wash.  Pub.  No.  195,  56  pp.,  3  pi. 

and  S.  Wright. 


1916.  Studies  of  inheritance  in  guinea-pigs  and  rats.    Carnegie  Inst.  Wash.  Pub.  241, 
192  pp.,  7  pi. 

Haldane,  J.  B.  S.,  A.  D.  Sprunt,  and  N.  M.  Haldane,  1915.    Reduplication  in  mice. 
Jour.  Genet.  5,  133-135.    [Reference  from  Detlefsen  (1918)  Genetics,  3,  p.  597.] 

King,  Helen  D. 

1918.  The  effects  of  inbreeding  on  the  fertility  and  on  the  constitutional  vigor  of  the 
albino  rat.     Jour.  Exp.  Zool.  26,  pp.  335-378. 

Punnett,  R.  C. 

1912.  Inheritance  of  coat-colour  in  rabbits.    Jour.  Genet.,  2,  pp.  221-238,  3  pi. 


Plate  i 


4  5 


10 


11 


4a  .^^rA- 


j^ 


V.i 


14 


J 


J 


12 


ir, 


16 


17 
Grades  1-ls  ot  Dutch  rahl^ils. 


IK 


PLATE  2 


10 


-Ml 


F- 


i 


Jl 


V"\ii.  I'.t,  .•!    ■  while"  Diil.li  r.iMiii.    9  7',i:;i,  ii\:\i\v  I.'.,     l"in.  L'O.  :i  "(liirk"  Diilrli  r.iMiii. 
o'iuOl,  gnulc  .').     I'ifi.  Jl.  .1  •t.in  "  Diilrh  r.il>l>it.   9717.  uniclc  :{. 


PLATF  :i 


l'"[(;s.  l-o.  Photographs  of  KiifiUsh  rabbits  adopted  :is  f;r;ulrs  l-.">iii  classifying  tlic  variatioii~ 
observed  in  the  Knfjhsh  pattern.  Those  shown  in  tigs.  1  and  J  wrrc  hoinozynons.  thosi-  shown 
in  figs.  :\  to  ")  were  heterozygous. 

Fk;.  ().  A  'hifili-grade"  homozynous  Eufihsli  ral)bit,  1  17.'5.  ^rade  3'2.  Kriielically  riini|viniblc 
with  a  firadc  .'>  hctcniwjioii^  l-]ntilisli.     Compare  Hn-  •''• 


v^ 


